142 How Living Things Come to be What They Are 



Development in Plants 



It has not been easy to apply the biogenetic law to 

 the development of plants, because the earlier stages in 

 the development of higher plants have come to be well 

 known only in more recent times. Moreover, in the earlier 

 stages of their development, plants do not manifest such 

 distinctive and characteristic structures as do animals, so 

 that comparison has been difficult. And finally the life 

 cycle of all plants above the mosses and liverworts shows 

 peculiarities that make impossible the kind of compari- 

 son which has been so fruitful in the study of animal 

 development. 



As knowledge increases, however, the same general 

 principles appear. Every plant individual starts life nor- 

 mally as a single cell. Cell division results in increasing the 

 number of cells making up the body mass. New structures 

 appear by elaboration and specialization of cells. The early 

 stages of the highly developed plants correspond in many 

 respects to the structure characteristic of lower types of 

 plants. In the lowest plants sexual reproduction takes place 

 in the water, as is the case in the siniplest animals. The 

 gametes (germ-cells, eggs and sperms) come together by 

 floating or swimming in the water. 



The liverworts, mosses and ferns live upon land, al- 

 though, like the amphibians among animals, they must return 

 to the water during part of the year in order to make fertili- 

 zation possible. This does not mean that mosses and ferns 

 move into the pond when spring comes, as do the frogs. It 

 means only that in these plants the stages during which sexual 

 reproduction takes place are low or flat and depend upon being 

 submerged by rain water, or even by a few drops of dew in 

 some cases. In this condition the sperm cell can swim toward 

 the enclosed egg cell. With higher plants the sexual phase 

 is reduced still further, and the parent plants manifest a great 

 variety of adaptations to the need for bringing the sperm and 

 ^§§ together. In the flowering plants the sperm floats in a 



