96 BOTANY 



sexual process of reproduction led to the appearance of 

 fewer offspring than the method of gonidia production, 

 but those so formed were much more vigorous and better 

 qualified in every way to carry out their vital processes, 

 and hence this process has become almost universal. 



This stage does not, however, mark finality among 

 plants. It involved a good deal of uncertainty as to 

 whether fertilisation would take place, as both the 

 parents of the sperm cells and the ova were unable to 

 move except as they were drifted about in the water. 

 The sperm retained the power of swimming when 

 liberated, but the ovum in the higher types remained 

 enclosed in the oogonium or cell in which it was formed. 

 This uncertainty led to a further development, by which 

 one act of fertilisation was made to produce several 

 young plants instead of one. This was brought about 

 by the zygote the fertilised female cell dividing up 

 into a number of cells, which were liberated by the 

 bursting of the zygote wall. These cells in most cases 

 were furnished with cilia and in general appearance and 

 behaviour were hardly if at all distinguishable from 

 the gonidia described before. Such reproductive cells 

 can be seen to-day in the alga (Edogonium. To distin- 

 guish them from the gonidia they are spoken of as 

 spores. One act of fertilisation results in this way in 

 the production of a number of new individuals. 



A little further advance still was made among the 

 algae, which became, however, much more marked 

 among the plants which established themselves on land. 

 Instead of the fertilised cell dividing at once to produce 

 the spores, it developed into a relatively bulky multi- 

 cellular structure, which became differentiated so that 

 only part of it gave rise to the spores, while the rest 

 served to protect them and to minister to their nutrition. 

 Such a structure can be seen to-day in many of the red 

 seaweeds, in which it is known as a sporocarp. 



