4/8 ELEMENTARY BIOLOGY 



On the reproductive side we may see a similar advance in 

 complexity from the lowest to the highest of these three plants. 

 In the Spirogyra every cell may act as a gamete, after behaving 

 for some time as a vegetative cell. In the moss certain special 

 cells are borne, in special organs, in a special region of the 

 vegetative plant. And the two gametes (male and female) are 

 borne on two different individuals. In the bean the gametes 

 are still more highly specialized, each being produced in a 

 very simple plant that is parasitic upon the parent. But these 

 two simple plants grow from very highly specialized structures 

 (embryo sac and pollen grain) that are in turn borne on very 

 highly specialized organs (pistils and stamens) which together 

 form a structure (the flower) that is almost completely devoid 

 of any vegetative behavior. In the flower we have an organ 

 that is specialized for producing bodies which have to do ex- 

 clusively with reproduction and the protection and distribution 

 of the next generation. 



502. The basis of classification. While it is possible to say 

 in a general way that a given plant is higher than another, it is 

 quite impossible to place all the known plants in a series from 

 the lowest to the highest. This would be as absurd as trying to 

 arrange all people in a series from the " worst " to the " best." 

 We find that there are several main divisions, some of which 

 we should place higher and some lower. But we find in each 

 division so many degrees of complexity that there is consider- 

 able overlapping when it comes to arranging all the plants. 



The first separation that we can make is one between plants 

 that bear seeds and those that do not. The seed plants can be 

 further divided into those that have closed carpels (pistils), like 

 all the flowering plants, and those that have open or exposed 

 ovules, like the cone-bearing plants. 



Among the non-seed-bearing plants there is a large group 

 in which the egg cell is borne in a special organ, the arche- 

 gonium (see pp. 320-321); this includes the ferns and their 

 allies, the mosses and the liverworts. 



