PHANEROGAMIA. 



397 



lar nutritive material in its cytoplasm, and has two coats, an 

 intine and an exine, the structure of the latter being elaborate in 

 many cases. The spore has 

 not, however, always two 

 coats. In some cases there 

 is no exine, and only a 

 single thin coat, as in the 

 cells of the pollinia of Or- 

 chids and Asclepiads, and 

 in certain plants whose 

 flowers develope under wa- 

 ter, such as certain Naiad- 

 aceae. In others, again, 

 there is but one coat, but 

 it is thick and is cuticular- 

 ised externally (e.g. Senecio), 

 or the two coats are only distinguishable at those points at which 

 the pollen-tubes will be eventually protruded (e.g. Onagracese). 



FIG. 235. A Pollen-grain of Cucwbita Pepo, 

 showing the lid-like areas through which the 

 pollen tubes will protrude ( x240). B Section of 

 one of these areas ( x 540) (after Strasburger). 



Fm. 236. Pollen-grains of Malva crispa. A Grain seen from surface ; B section of wall, 

 showing the exine with its alternate spines and pores, the latter closed internally by the 

 delicate innermost layer of the exirie ; C germinating pollen-grain with pollen-tubes; D the 

 same in section, showing the protrusion of the pollen, tubes through the pores. (A, B,D 

 XalO; Cx 240-: after Strasburger.) 



