separate, and so the most highly organised plant becomes possible. This is seen among the Green Algae where the cells are often joined end to end in filaments. Everyone is acquainted with these humble plants which wave like green hairs in the water. Among the lower Green Algae the differentiation of cells is not very great. A good many multicellular forms have all their cells the same in structure and function and are spoken of as physiologically unicellular.

In the higher forms of Algae there is, however, differentiation in two directions. First the cells which attach the plant to a substratum contain less chlorophyll and take a peculiar form. In this way a rudimentary fixing organ is formed. Secondly there is a differentiation between the supporting axis and the appendages. The cells of the axis contain less chlorophyll and become stouter. It is perhaps needless to point out the likeness to root, stem, and leaf in the higher plants. The simplest organism must adapt itself to the needs of its environment, and the division of physiological labour by slow degrees produces more complex organisation.

The Green Algae are for the most part fresh-water plants. They are, as a whole, of very simple structure; but it is amongst them that one must look for the ancestors of the lowest green land-plants, the Liverworts. There is reason for thinking that these originated from aquatic plants allied to certain existing Green Algae. The most striking fact when one compares these simple plants with the higher forms is the more and more marked specialisation of parts to fit them for special functions. At a very early stage among the lower plants one finds a modification of certain cells for reproductive purposes. In the earliest phases these reproductive cells are hardly to be distinguished from vegetative ones. Later they become extremely modified and can only serve for reproduction. The plant reproduces itself sexually when two sorts of cells are formed neither of which can develope further except as the result of a union of the two. It is probable that the stimulus required to produce developement is a chemical one. Among the Green Algae, sexual as well as asexual reproduction may be clearly distinguished. When such plants are asexually reproduced a specialised cell is formed which is endowed with the power of growing into a new structure. These cells are capable of some endurance and correspond with the familiar brown spores on the back of a fernleaf. In sexual reproduction, the plant also forms a specialised cell, but this is the result of the union of two other cells. These cells, as we have already seen, are called gametes. Among the lower plants the two gametes are frequently similar in

appearance. But as we go up in the scale a female gamete or egg-cell is distinguishable under the microscope from a smaller and more active male cell, which consists possibly of nothing more than a nucleus. When gametes, or sexual cells, are formed in Green Algae the male is provided with cilia or motile hairs with which it swims through the water until the two meet and fuse.

It is of the greatest importance to understand the difference between sexual and non-sexual reproductive cells, because in the life-history of all plants higher than the Green Algae we find alternating phases or generations. The one produces asexually formed reproductive cells and is called, as the reader doubtless remembers, the sporophyte or spore-bearing plant; the other produces sexually formed cells and is called the gametophyte or gamete-bearing plant. We have seen this in the ferns. Among the Green Algae the sexual, or gametophyte phase, and the asexual, or sporophyte phase, may be distinguished, but the two forms do not succeed one another in clearly alternating generations. This alternation of generations is believed, as we shall see, to be connected with the change from aquatic surroundings to life on dry land.

The vegetative as well as the reproductive cells of plants are, also, found to be differentiated as we pass from the lower to the higher forms. While in the unicellular plant the same cell serves to perform all functions, in the higher plants special organs are developed for special purposes. Not only is the plant-body multicellular, but the cells show great variety of form and structure. Tissues of different kinds are produced out of which special organs are formed. There is a subterranean root system for fixing the plant and supplying it with liquid nourishment. The green assimilative tissue is mainly confined to the leaves, which are organs for extracting carbon from the carbon dioxide of the atmosphere. The highest specialisation is reached in the flowers of seed-bearing plants. The elaborate machinery by which pollen is dusted on a bee and so transferred to other flowers is known to everyone.

The origin of species by descent with modification is now universally accepted by botanists. Overwhelming evidence shows that all the various kinds of plant have been produced from predecessors which differ from them comparatively slightly in the later stages of time. But these predecessors become simpler and less like their descendants as we go further back. So we reach the simplest plants. Since it is believed that Liverworts, Mosses and all the higher vascular plants are descended from algal ancestry, it is natural to suppose that

long before the luxuriant plant growth of the Carboniferous period there must have existed an age of Algae. Fossil Algae are found, but they have added little to our knowledge so far, and many remains have been described as Algae on very inadequate evidence. It is not surprising that they are not better preserved as fossils if we remember how delicate and destructible their cellular tissue generally is. With higher plants, and especially with ferns, horse-tails, club-mosses and cycads it is a very different matter. The discoveries which fossil botanists have made during the last dozen years have altered the whole aspect of the subject. In many fossil plants the cells themselves are petrified. Sections can be prepared for the microscope which show in a wonderful fashion details of the cell-wall and even the existence of the nucleus. The student of evolution finds his knowledge enormously added to by a study of fossil plants, and paleobotany is still almost in its infancy. Unfortunately most that has been written on this subject is unintelligible to the untrained botanist. Ancient 'Plants,' by Dr. Marie Stopes, gives a fairly simple account of the subject, and it is abreast of modern discoveries. It may be recommended to the general reader whose interest in science is sufficiently ardent to make him master a good many alarming botanic terms when their meaning has been explained. Professor Campbell's Lectures on the Evolution of Plants' is an American work written in an untechnical manner from which the general reader may learn much that will be interesting. It is clearly worded, fairly well illustrated and scientifically arranged, but unfortunately parts are now antiquated and the book is consequently occasionally untrustworthy. It is nevertheless well worth reading. The remaining works which we have placed at the head of this article are written for serious students and specialists.

We pass now from the Algae, which are aquatic. The first group of mainly terrestrial plants at once shows tissue differentiation in response to the conditions of life on land. Yet the lowest Liverworts have an extremely simple vegetative structure. It is little more advanced than that found in some of the higher Green Algae. But before going further it may be well to remind the reader, who is perhaps not familiar with classification, of the main groups into which plants are divided by botanists. It is unfortunately impossible to follow anything that has been written on the evolution of plant life without understanding the chief distinctions between Thallophytes, Bryophytes, Pteridophytes, and Spermophytes.

It may be convenient for readers who are not botanists to have

before them in tabular form the names of the chief groups of plants.*

The Thallophytes are the lowest or simplest plants and comprise, besides the Algae, two other groups which need not trouble us further in considering the origin of land-plants. These two other groups are the Bacteria and the Fungi. Neither of them shows ancestral affinity with the higher green plants. The fungi are thought to be the degenerate descendants of green ancestors. They are all parasites or saprophytes, which feed on living or dead organic bodies respectively, and are themselves destitute of chlorophyll. The Algae, on the other hand, all contain chlorophyll. Many seaweeds are highly specialised and of great size, but there is no evidence that they have given rise to any higher types of plant structure.

The Bryophytes comprise the Liverworts and Mosses. They are terrestrial but moisture-loving forms, though many can be completely dried up and revive without injury. In all of them there is a regular alternation of generations. The liverwortplant and moss-plant, with which everyone is familiar, is the gametophyte, or generation with sexual organs, producing gametes. The fusion of the gametes produces a sporophyte which remains attached to the parent plant and produces spores. But so far the gametophyte is the conspicuous generation.

The Pteridophytes include Ferns, Horse-tails and Club-mosses in all of which the two generations, sporophyte and gametophyte, are clearly distinguished. But here for the first time the sporophyte is the more conspicuous. The fern-plant with which everyone is familiar bears spores on the back of its leaves. When these fall to the ground and germinate the gametophyte is produced bearing sexual organs producing gametes. It is, as we have seen, small and insignificant, and when the gametes have

fused it withers and the sporophyte flourishes quite independently of the gametophyte. So the cycle of life proceeds. It is very significant that throughout the Liverworts, Mosses, Ferns, Horse-tails, and Club-mosses water should be essential to the gametophyte or sexual generation for fertilisation. Water is needed for swelling and opening the sexual organs, and the liberated male-cells then swim in the water to fuse with the eggcell. At this stage the gametophyte becomes in effect an aquatic plant and seems to revert to the condition of its aquatic ancestors. The Spermophytes are the seed-bearing plants. Some of them are also called flowering plants, but the essential point about them is the seed which follows upon the flower. In this group there is also an alternation of sexless and sexual generations. As in the Pteridophytes, the sporophyte is the conspicuous generation. It is the sporophyte which we see in all seed-bearing plants, such as the pine-tree, the humble daisy, or the massive oak. The gametophyte is so inconspicuous and so deeply imbedded in the tissues of the sporophyte that its existence is ignored by the ordinary person. It will be easier to explain this in greater detail later. For the moment it is enough to remember that even in the highest plants there is an alternation of generations, though the gametophyte is almost suppressed. The sporophyte produces spores which grow into the gametophyte. So, as we have said of the other groups of plants, the cycle of life proceeds.

It is important before going further to bear in mind the essential distinction between spores and seeds. A spore is a unicellular body formed asexually and set free from the parent plant for the purposes of reproduction. It produces a gametophyte. A seed is a multicellular body, the important part of which is formed sexually by the fusion of gametes. It is not set free from the parent plant until an embryo, or young plant, has developed within it. When it germinates it produces a sporophyte. The Spermophytes, or seed-bearing plants, fall into two divisons, Gymnosperms and Angiosperms. The Gymnosperms have naked seeds and include the Conifers and the Cycads. The Angiosperms have seeds in a closed seed-case and include all the flowering plants with which we are most familiar. But although the two divisions have the habit of seed-bearing in common, recent research has proved that the Gymnosperms are separated by a greater gulf from Angiosperms than they are from ferns and other Pteridophytes. One of the triumphs of palaeobotanists has been the discovery of an intermediate fossil group of seed-bearing ferns.