EVOLUTION OF THE LAND PLANT 97 



In the land plants, as we shall see presently, this line 

 of development became much more extended. The 

 zygote gave rise to a much more highly organised sporo- 

 carp, in which the cells which formed the spores became 

 more restricted in their numbers and disposition. The 

 further progress of development led to much greater 

 differentiation of the sporocarp, which gave it the power 

 of living independently. The sterile part, or the region 

 which did not directly form spores, became much en- 

 larged and grew to dimensions much exceeding those of 

 the original thallus. Its body became differentiated 

 into root, stem, and leaves, and upon the sub-aerial 

 parts the spores were formed in structures known as 

 sporangia. This structure, known as the sporophyte, is 

 now the dominant form of all terrestrial plants. We 

 shall consider this change in the next chapter. 



CHAPTER XIII 



THE ORIGIN OF TERRESTRIAL PLANTS EVOLUTION 

 OF MOSSES AND FERNS 



WE must now consider what has been perhaps the most 

 important step we find in the development of vegetation, 

 the transference of plants from water to land. At first 

 all were aquatic, but in the natural course of events no 

 doubt many were washed on to the moist earth by the 

 side of the water in which they were living. Being 

 adapted only to life in water no doubt most perished, 

 and it was only gradually that some established them- 

 selves. A comparison of aquatic with terrestrial forms 

 as we find them to-day shows how complete a change 

 in almost every respect took place. In the water the 

 direction of their growth was comparatively unimportant, 

 and a large number grew in a horizontal position. On 

 land we find this extremely rare; most plants, as we 



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