3i6 THE BIOLOGY OF FLOWERING PLANTS 



embryo sac. The gametophyte generation is reduced to 

 three cells in the pollen grain, and to eight in the embryo 

 sac, including the gametes in each case. Of the two male 

 gametes one fuses with the egg cell, and forms the zygote, 

 which immediately develops into an embryo, the early stage 

 of a new sporophyte generation. The gametophyte genera- 

 tion in the angiosperms is thus reduced to a very few cells 

 and has only a transient existence. In the gymnosperms it 

 is rather better developed, but in these, too, it passes its 

 existence entirely within the parent spore. Only by 

 comparative studies with lower types has it become 

 perfectly clear that we have, in the flowering plants, a 

 definite alternation of generations of the same type as that 

 found so much more obviously in the mosses and ferns. 



In the moss the gametophyte is the moss plant, the 

 sporoph5^e is the capsule with its stalk. The sexual genera- 

 tion is the dominant one. The fern plant is a sporophyte, 

 but the gametophyte leads an independent though modest 

 existence, as the prothallus. As we follow the changes 

 upwards through the evolutionary scale we find the gameto- 

 phyte becoming more and more dependent. In Selaginella, 

 a heterosporous lycopod, the female gametophyte just 

 protrudes from the bursting spore. In the gymnosperms 

 it is entirely contained within the spore, and, further, the 

 megaspore remains embedded in the tissue of the sporan- 

 gium, the nucellus, nourished and protected throughout 

 by the parent plant. In the Cycads and in Gingko we still 

 find free-swimming ciliate sperms, but already in the higher 

 gymnosperms the sperm has lost its power of independent 

 locomotion and is carried by the pollen tube to the embryo 

 sac. At the most it may have some power of amoeboid 

 movement there. The cases mentioned are not stages in 

 one phylogenetic line, but they may be taken as illustrating 

 steps in the reduction of the gametophyte generation. 



This process of reduction has taken place during the 

 evolution of a flora more and more suited to life on a land 

 surface, and the advantage of a reduced and dependent 

 gametophyte to a land plant is easily understood. The 



