21 



Shortly after deposition of self- or wind-borne pollen 

 upon the stigma, a tube is protruded through a specialised 

 germ pore in the extine or outer wall of the pollen grain. 

 It grows down an intercellular space between the 

 four rows of cells composing one of the feathery branches 

 to the inner conducting tissue of the style as in rye (Fig. 

 22). On reaching the ox'ary cavity the tube bends round 

 and takes a course between the inner wall and outer 

 integument of the ovule till it reaches and enters the 

 micropyle, which has already been forced open by the 

 adjacent egg-apparatus nuclei (Figs. 24 and 2^). 



FERTILISATION. 



The pollen-tube passes into one of the synergids and 

 discharges its contents (Fig. 25). Although double 

 fertilisation could not be demonstrated, there is 

 undoubtedly the suggestion that it does take place in the 

 prompt appearance of the endosperm. Guignard^' 

 described Zca, Indian corn, as being " double " fertilised 

 and remarked that he was unable to follow the course of 

 division in the endosperm nucleus, as this took place so 

 rapidly. The foremost generative nucleus probably unites 

 with the two extra-polar nuclei to form the nucleus that 

 initiates this formation of endosperm. Its simultaneous 

 mitoses soon produce a layer of nuclei embedded in dense 

 granular cytoplasm, and this lines the sac wall on all sides. 



The second generative nucleus effects fertilisation 

 with the egg, and their fusion seems to be very quickly 

 consummated, for the zygote produced soon shows a single 

 central nucleus lying in a dense reticulum of deeply 

 staining cytoplasm (Fig. 26). 



EMBRYOGENY. 



The fertilised egg occupies a cup-shaped depression 

 in the apical part of the endosperm. Its divisions to form 



