REPRODUCTION AND DEVELOPMENT 257 



spores are commonly known as pollen. By one means or another 

 pollen is transferred from the anthers to the stigma at the upper 

 end of the pistil. Here the microspore germinates and produces the 

 microgametophyte; it consists of but three cells. One of these de- 

 velops a long tube that passes down the pistil into a chamber con- 

 taining an ovum. The sperm nucleus traverses this tube and 

 fertilizes the ovum. The walls of the chamber and of the base of the 

 flower become modified to form the seed coats and parts of the 

 fruit at the same time the fertilized egg within is developing. De- 

 velopment proceeds until the fundamentals of a root and shoot 

 system appear, when development stops. Thus a seed contains an 

 embryo sporophyte in an arrested condition, together with a certain 

 amount of reserve food material. 



Here in the Spermatophyta is a separation of the spore-producing 

 organs of the sporophyte into those that produce egg-forming, and 

 those that produce sperm-forming gametophytes, a condition known 

 as HETEROSPORY. In most plants both types of spores develop in the 

 same flower, but their separation is not unusual. In the common 

 corn, maize, for instance, the silk represents the pistils of many 

 compartments containing egg-producing macrospores, forming 

 megagametophytes, while the dust of the tassel is made up of 

 microspores, which when they germinate produce the sperm-yield- 

 ing microgametophyte. The whole series from the Bryophyta 

 through the Pteridophyta to the highest flowering plants shows a 

 great reduction in the relative size of the gametophyte generation, 

 but it is not abolished completely. Metagenesis is therefore the order 

 of the life cycle of three of the four plant phyla. 



Metagenesis in Animals. The common nature of life proc- 

 esses among plants and animals is emphasized by the fact that 

 among animals one also finds metagenesis. Alternation of genera- 

 tions in the life history of the malarial parasite has been described 

 elsewhere (p. 75), It is sufficient here to call attention to the occur- 

 rence of this characteristic in some of these single cell animals. 



