98 BOTANY 



have seen, grow vertically upwards and downwards, and 

 show very different peculiarities in the parts which take 

 these opposite directions. The transference probably 

 did not take place till the habits of reproduction we 

 have briefly described had been acquired. Most likely 

 the first form which secured a footing in the soil was a 

 flattened thallus, consisting perhaps of only a few cells. 

 Its small size would enable it to touch the moist earth 

 over a large part of its surface and so to be able to 

 absorb the water it required. But with multiplication 

 of cells, and consequent increase of size, this became dim- 

 cult or impossible. It could only reach the moisture in 

 places and the supply so became insufficient. We find 

 such plants now on the earth, and we see that to secure 

 the water supply they have developed outgrowths of 

 their surface cells which resemble root hairs in struc- 

 ture; these the rhizoids adhere very closely to the 

 surface and gradually penetrate between the particles of 

 earth, so burying themselves in the soil. They become 

 very closely attached to its particles, so that they are 

 able to absorb the film of water by which each particle 

 is surrounded. They are produced in great numbers 

 and are continually being renewed. It is probable that 

 it was by such an arrangement that the early terrestrial 

 plants were enabled to establish themselves. 



The difficulties which they encountered caused them 

 to produce as many and as vigorous offspring as possible, 

 and as in consequence of the scarcity of water fertilisa- 

 tion became more and more difficult, the sperms not 

 being brought into the neighbourhood of the ova, they 

 gradually came to develop an increasing number of 

 spores from each fertilised cell. This was advantageous 

 in another way as the spores were more capable of resist- 

 ing the adverse conditions than their parent, owing to 

 their simpler structure, their more moderate require- 

 ments, and their thicker outer membrane. On the 



