Ancient Plants / Being a Simple Account of the past Vegetation of the Earth and of the Recent Important Discoveries Made in This Realm of Nature

by Marie Carmichael Stopes

This, however, as a byword. Notwithstanding these difficulties, which it would be unfair to ignore, the main scheme of evolution stands out clearly before the scientist of to-day, and his views are largely supported by many important facts from both fossil and living plants.

Very strong evidence points to the conclusion that the most primitive plants of early time were, like the simplest plants of to-day, water dwellers. Whether in fresh water or the sea is an undecided point, though opinion seems to incline in general to the view that the sea was the first home of plant life. It can, however, be equally well, and perhaps even more successfully argued, that the freshwater lakes and streams were the homes of the first families from which the higher plants have gradually been evolved.

For this there is no direct evidence in the rocks, for the minute forms of the single soft cells assumed by the most primitive types were just such as one could not expect to be successfully fossilized. Hence the earliest stages must be deduced from a comparative study of the simplest plants now living. Fortunately there is much material for this in the numerous waters of the earth, where swarms of minute types in many stages of complexity are to be found.

The simplest type of plants now living, which appears to be capable of evolution on lines which might have led to the higher plants, is that found in various members of the group of the Protococcoideæ among the Algæ. The claim of bacteria and other primitive organisms of various kinds to the absolute priority of existence is one which is entirely beyond the scope of a book dealing with fossil plants. The early evolution of the simple types of the Protococcoideæ is also somewhat beyond its scope, but as they appear to lie on the most direct “line of descent” of the majority of the higher plants it cannot be entirely ignored. From the simpler groups of the green Algæ other types have specialized and advanced along various directions, but among them there seems an inherent limitation, and none but the protococcoid forms seem to indicate the possibility of really high development.

Fig. 17.—A Protococcoid Plant consisting of one cell

p, Protoplasm; n, nucleus; g, colouring body or chloroplast; w, cell wall.

In a few words, a typical example of one of the simple Protococcoideæ may be described as consisting of a mass of protoplasm in which lie a recognizable nucleus and a green colouring body or chloroplast, with a cell wall or skin surrounding these vital structures, a cell wall that may at times be dispensed with or unusually thickened according as the need arises. This plant is represented in [fig. 17] in a somewhat diagrammatic form.

In such a case the whole plant consists of one single cell, living surrounded by the water, which supplies it with the necessary food materials, and also protects it from drying up and from immediate contact with any hard or injurious object. When these plants propagate they divide into four parts, each one similar to the original cell, which all remain together within the main cell wall for a short time before they separate.

If now we imagine that the four cells do not separate, but remain together permanently, we can see the possibility of a beginning of specialization in the different parts of the cell. The single living cell is equally acted on from all sides, and in itself it must perform all the life functions; but where four lie together, each of the four cells is no longer equally acted on from all sides. This shows clearly in the diagram of a divided cell given in [fig. 18]. Here it is obvious that one side of each of the four cells, viz. that named a in the diagram, is on the outside and in direct contact with the water and external things; but walls b and c touch only the corresponding walls of the neighbouring cells. Through walls b and c no food and water can enter directly, but at the same time they are protected from injury and external stimulus. Hence, in this group of four cells there is a slight differentiation of the sides of the cells. If now we imagine that each of the four cells, still remaining in contact, divides once more into four members, each of which reaches mature size while all remain together, then we have a group of sixteen cells, some of which will be entirely inside, and some of which will have walls exposed to the environment.