THE DEVELOPMENT OF THE PLANT 57 



of plants had preceded them in the history of the earth. 



What the line of development was up to these large 

 marine Algae we can only conjecture by arranging in a 

 series the lower plants that we find in nature to-day. 

 We have tiny plant-cells (like Chroococeus] living separate 

 lives of the utmost simplicity : we have next cells of the 

 same type living in a common jelly-like deposit (as in 

 Aphanocapsa) : in Glcelocapsa the cells come closer 

 together: in Volvox they form definite and orderly 

 structures, making a multicellular (many-celled) organ- 

 ism. This was undoubtedly the way in which the 

 primitive single-celled organisms came to form composite 

 (or multicellular) bodies ; but we will return to this 

 point in the next chapter. A dip in almost any old rain- 

 gutter will bring up specimens of each stage in the 

 process. 



The more interesting point is to see how these simple 

 Thallophyta, as the botanist calls them, lead on to our 

 familiar mosses, ferns, and flowering plants; and this is 

 by no means easy. Let us first read the story as it is 

 suggested by the geological record. By the Silurian 

 period the waters of the ocean swarmed with Algae, from 

 the single microscopic cell to the large branching sea- 

 weeds that grew up from the floor of the sea. The land 

 was meantime rising above the surface of the water, and 

 on some shallow shore or in some evaporating lake the 

 plant adapted its structure to life on land. We shall 

 see a more interesting adaptation of that kind when we 

 come to deal with animal evolution. Before the end of 

 the Silurian period we find traces of land vegetation, 

 "and we can," says one of our leading geologists, " dimly 

 picture the Silurian land with its waving thickets of 

 fern, above which lycopod trees raised their fluted and 

 scarred stems, threw out their scaly moss-like branches, 

 and shed ther spiky cones." 



