382 CHARACTERS OF THE HIGHER PLANTS LESS. 



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 (t l , / 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 embedded 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. 376). Of these 

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

 synergidce (sng) : the third, more deeply placed, is the ovum 

 (ov). The two unaltered nuclei now travel to the centre of 

 the megaspore and unite with one another, forming the 

 secondary nucleus (nu) of the spore. 



There is thus a single ovum produced in each megaspore, 

 but no ovary and no prothallus : the female portion of the 

 gamobium is reduced to its simplest expression. 



Pollination may take place, as in Gymnosperms, by the 

 agency of the wind, but usually the microspores are carried 

 by insects, which visit the flowers for the sake of obtaining 



