ORGANS OF REPRODrCTION. 



265 



mose still goes on, the intine Avill itself burst and discharge the 

 contents of the pollen-grain in the form of a jet (Jig. 560), 

 These changes take place more rapidly, if a little sulplmric or 

 nitric acid be first added to the water. 



When the pollen is thrown upon the stigma under natural 

 circumstances at the period of the dehiscence of the anthers, 

 the aboA-e-described action becomes materially modified. Here, 

 the pollen-grain does not burst, but its intine' protrudes through 

 one or more of the pores or slits of the extine in tlie form 

 of a delicate tube (figs. 561 and 562) filled with the fovilla, 



Fig. 560. 



Fig. 561. 



Fig. 560. rollen of Cherry discharging its fovilla through an opening in the 



intine Fig. 561. Trigonal pollen of ^/iof^era with a pollen-lube -• 



Fig. 562. Vertical section of the stigma and part of the stj'le of Antirrhi- 

 num mojus. stig. Stigma, on which two pollen-grains have fallen, each of 

 which is provided with a pollen-tube, tp, wliich pierces the tissue of the 

 style, styl. 



and called, the pollen-tube; this penetrates (as will be afterwards 

 described^ through the tissue of the stigma and style to the 

 ovules. This tube is frequently some inches in length, and its 

 formation is not due as was formerly supposed to endosmotic 

 action, but it is a true growth like that of a seed genninating, 

 and caused by the nourishment it derives from the stigma and 

 style. 



2. The Disk. 



By the disk, we understand all bodies of whatever fonn, 

 which are situated on the thalamus between the stamens and 

 the ovary, but which cannot be properly referred to either 

 of those organs. This is the sense in which it is properly ap- 

 plied, but by some botanists the term is understood as synony- 

 mous with torus, and defined as that part of the receptacle 



