378 ANGIOSPERMS 



the form of a slender rod-like body, the style (st), terminated 

 by an enlarged extremity, the stigma (stg) which is covered 

 with hairs and is frequently sticky. In some flowers, on the 

 other hand, all the carpels of the gynsecium unite with one 

 another by their adjacent edges, so as to inclose a cavity 

 common to all : in this case also the hollow portion or venter 

 is formed by the proximal part only of the carpels, their 

 distal portions forming a simple or multiple style and 

 stigma. 



The megasporangia (ovules, A and B, mg. spg) are usually 

 borne on the edges of the carpels, and, owing to the union 

 of the latter, become inclosed in the cavity of the venter, 

 and are thus completely shut off from all direct communica- 

 tion with the external world. It is this inclosure of the 

 megasporangia in a cavity formed by the sporophylls on 

 which they are borne which constitutes the chief character 

 distinguishing Angiosperms from Gymnosperms. 



The megasporangia (D) differ from those of Gymnosperms 

 chiefly in having a double integument : both coats (/ a , t 2 ) as 

 well as the nucellus (nd\ or central mass of tissue, are com- 

 posed of small cells : and the megaspore (embryo-sac mg. 

 sp) is a single cell of great size imbedded in the nucellus. 



No prothallus is formed in the megaspore, but its nucleus 

 divides, the products of division pass to opposite ends of the 

 spore, and each divides again and then again, so that four 

 nuclei are produced at each extremity. Three of the nuclei 

 at the proximal end that furthest from the micropyle 

 become surrounded by protoplasm and take on the character 

 of cells (D, ant} : the fourth remains unchanged. Similarly 

 of the four nuclei at the distal or micropylar end, one 

 remains unchanged and three assume the form of cells by 

 becoming invested with protoplasm (see p. 373). Of these 

 three, two lie near the wall of the megaspore and are called 



