EVOLUTION OF THE LAND PLANT 99 



other hand, the multiplication of the young plants to 

 which the spores gave rise made competition much more 

 severe. The young plants were produced so close to 

 each other as to cause great overcrowding, making it 

 difficult for each to be properly illuminated and to 

 secure sufficient nutritive material from the soil. 



The flattened form of plant under these conditions 

 became unsuitable, and each plant was compelled to 

 grow upwards or perish. The rhizoids in this way 

 tended to accumulate on the comparatively small part 

 of the thallus which was left nearest to the soil so that 

 the rest might raise itself into the air. Gradually thus 

 the plant came to show the descending portion adapted 

 to live buried in the soil and the ascending portion 

 freely exposed to light and air. We find thus an indica- 

 tion of the parts we have spoken of as the root and the 

 shoot. It must not be concluded, however, that these 

 early plants showed either in such a state of development 

 as we recognise in the land plants of to-day. 



The methods of reproduction gradually underwent 

 much modification, as those of the aquatic plants were 

 but slightly suitable for erect plants, only part of which 

 could have free relations with water or moist earth. 

 The change of position rendered fertilisation uncertain, 

 and it became more and more difficult for the sperms to 

 get to the ova. To help the process to take place, more 

 elaborate structures were formed in which to develop 

 the sperms and the ova. These, known as antheridia 

 and archegonia, were produced in the neighbourhood of 

 moisture as far as possible, often on the under side of 

 the flattened thallus, or, in the case of an erect form, on 

 the parts liable to be moistened by rain or dew. The 

 result of the difficulties associated with fertilisation 

 was a great development of the structure to which the 

 fertilised ovum gave rise, which produced large numbers 

 of spores. Gradually this became more and more 



