DEVELOPMENT OF SEAWEEDS 95 



was supplemented by another method which gave rise 

 to a renewal of vigour in the offspring. How the new 

 method was first brought about it is hard to say. It 

 can still be seen to play its part among the free swim- 

 ming gonidia of the seaweeds. Two of them, instead of 

 developing into two plants, fuse completely together to 

 form a single protoplast, which after a period of rest 

 grows into a single new plant. This union of the two is 

 called conjugation and the fused product is a zygote. 



We have in this process an indication of the way in 

 which sex in plants began to be developed. We cannot 

 speak of either of the two conjugating cells in this stage 

 as male or female, but the further development into 

 well-recognisable sexes can be easily traced. One of the 

 cells of the pair became larger and more sluggish than 

 the other ; then a stage was reached in which the larger 

 of the two was not motile at all. By and by, instead of 

 being developed from any cell of the plant they came 

 to be produced in particular cells or groups of cells 

 organs for their development. The smaller active cells 

 came to be recognised as male, the sluggish ones as 

 female. At first they were equally numerous, but as the 

 females became larger fewer of them were formed in the 

 specialised cells. The number varied indeed inversely 

 as the size. While the males continued to be produced 

 in large numbers the females became ultimately solitary 

 in the organs bearing them. Finally the female never 

 escaped from the organ but was joined there by a male. 

 The act of fusion of the two has come to be spoken of as 

 fertilisation. The female cell is now known as an ovum ; 

 the male cells are called sperms. There is an almost 

 infinite variety in the modifications which these lowly 

 plants have shown and show to-day in the ways they 

 develop their gonidia and their ova and sperms. We 

 cannot go more deeply into the matter here. 



From the elaborate nature of its mechanism, this 



