284 D. T. MACDOUGAL 



vegetative axis, and is therefore not directly derived from the primi- 

 tive anchorage organs described. 



The present occasion does not warrant a discussion of the evolu- 

 tionary development of the vegetal organism from the colloidal mass 

 to the gametophyte, now represented by the prothallium of ferns 

 and their allies. Neither is it necessary to recall details of plant 

 anatomy further than to point out that the earlier forms of plants, 

 co-ordinately with the monotonous conditions offered by their ac- 

 quatic habitats, showed no differentiation of tissues comparable 

 with that of the axis of the modern seed-plant, and that their 

 flattened bodies were for the most part closely appressed or adherent 

 to the substratum. The development of the sexual type of reproduc- 

 tion in such forms had been followed by a habit of formation of the 

 sexual organs separately, perhaps some distance apart on the upper or 

 lower surfaces of the body. In the functionation of such organs the 

 two kinds of protoplasts representing the sexual elements would be 

 set free at the surface of the body and accomplish union while swim- 

 ming freely, or in higher stages of development, the one representing 

 the egg-cell would remain in place, while the fertilizing protoplast, 

 or spermatozoon would find its way to it. In either case free water 

 was absolutely necessary for reproduction. The body of the plant 

 might be partially or completely immersed, or it might have only a 

 thin film coating the surface, through which the sexual elements must 

 move, but in either case the plant could not survive away from the 

 margins of streams, seas, and lakes, or up out of the moist lowlands, 

 or beyond the borders of rainy regions. 



The thallose forms carrying on sexual reproduction do not appear 

 to have been capable of the morphological development which might 

 have gained them independence from the water, and this freedom 

 was gained only after a secondary, asexual generation came into 

 existence. 



In the general movement which finally resulted in a land flora, 

 the fertilized egg held in the body of the thallus would germinate 

 in place, developing into a vegetative structure (the sporophyte) 

 unlike the thallus which bore the egg. Then cells were cut off, or 

 separated from the body of this alternate generation, known as the 

 sporophyte, which had the power of developing into thalli like the 



