POLLINATION AND FERTILIZATION 379 



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 (mi) 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 

 nectar, a saccharine fluid secreted by certain parts. The 

 microspores are deposited on the stigma (A), where they 

 germinate, each sending off a pollen-tube (A and c 2 , /. /), 

 which grows downwards through the tissue of the stigma and 

 style to the cavity of the venter, where it reaches a megaspo- 

 rangium, and, entering at the micropyle (D, /. /), continues 

 its course through the nucellus, finally applying itself to the 

 distal end of the megaspore in the immediate neighbourhood 

 of the synergidae. 



In the meantime the nuclei of the microspore (c 2 , nu, 

 nu'} have passed into the end of the pollen tube. They do 

 not become converted into cells as in Gymnosperms (p. 373) 

 but one usually remains undivided, while the other divides 

 by karyokinesis. 



Soon after this the ovum is found to contain two nuclei 

 instead of one, the second being the male pronucleus, derived 

 apparently from the nucleus of the smaller cell (c 1 , a) of the 

 microspore. It has been shown that one of the synergidae 

 performs the function of transmitting the male nuclear 

 material from the pollen tube to the ovum, but the precise 

 steps of the process are still uncertain. 



The two nuclei (male and female pronuclei) unite, and the 

 ovum, acquiring a cell-wall, becomes the oosperm or uni- 



