DOMAIN OF EVOLUTIONARY HYPOTHESIS 127 



tissue in addition (endosperm) in which several ovum 

 cells can be deposited (Fig. 28). In the Angiosperms 

 there still remain three cells, which may be regarded 

 as a prothallus (the so-called three antipodes). There 

 is never more than one ovum formed (Fig. 29). 



The determination of classes 

 within these series is effected 

 mostly according to the construc- 

 tion and the position of the spores 

 or the ovum and seed vessels, 

 the formation and arrangement of 

 the leaves, the small or tree-like 

 constitution of the stem (stalk), 

 etc. From all this there results a 

 habit of growth which is fairly 

 characteristic but can only be suf- 

 ficiently known by seeing the 

 plants. 



As an example we will describe 

 the classes of the Pteridophytes 

 (after Warming). 



Class 1 : True Ferns (Filicinse) 

 and Hydropteridse (Water Ferns). 



(a) Habit mostly herbaceous ; 



leaves alternate, large in comparison with the stem and 

 highly developed (feathery) ; when young, circinately 

 rolled (mostly). 



(b) Peculiarities of the reproductive organs : the 

 spore-cases are situated on the edge or on the back of 

 ordinary leaves ; only in some cases are the fruitful 



FIG. 29. E t Fruit bud 

 of an Angiosperm (Poly- 

 gonum convolvulus) dur- 

 ing fertilization. Pol- 

 len grains and pollen 

 tubes as above, e, em- 

 bryo sac ; E, the one 

 ovum cell (the two 

 others do not become 

 fertilized). A, the three 

 antipodes (=pro- 

 thallus). 



(After Strasburger.) 



