224 



THE FLOWER 



must not be confounded with it.) The pollen tube con- 

 tinues to elongate until it passes down through the base of 



the style into the ovary (Fig. 



443, in\ 



326. Course of the Pollen Tube. 



— The time taken for the tube to 

 penetrate to the ovary varies in 

 differ.ent flowers 

 according to the 

 distance traversed 

 and the rate of 

 growth. In the 

 crocus it takes 

 from one to three 

 days, in the spot- 

 ted calla i^Arinn 

 maculatum), about 

 five days, and in 

 orchids, from ten 

 to thirty days. In 

 the hibiscus and 

 many others of 

 the mallow family, we know that it 

 can not well exceed twenty-four 

 hours, as the corolla usually falls 

 away on the evening of the day on 



443. — Diagram of a simple 

 flower, showing course of tlie 

 pollen tube : a, transverse 

 section of an anther before its 

 dehiscence; b, an anther de- 

 hiscing longitudinally, with 

 pollen; c, filament; d, base of 

 floral leaves ; e, nectaries ; /, 

 wall of carpels ; ff, style ; h, 

 stigma ; 2, germinating pollen 

 grains ; m, a pollen tube which 

 has reached and entered the 

 micropyle of the ovule ; n, 

 funicle of ovule ; 0, its base ; 

 /, outer integument; g, inner 

 integument ; s, nucellus of 

 ovule ; /, cavity of the embryo 

 sac ; u, its basal portion with 

 antipodal cells; v, synergidas; 

 z, oosphere. 



444. — . 



A pollen 

 grain emittmg a tube 

 (magnified). 



which it expanded, carrying the 

 style and stamens with it, so that if the pollen tube had 

 not reached the ovary by that time it could never get 

 there at all. Sometimes the pistil is hollow, affording a 

 free passage to the pollen tube ; in other cases it is solid 

 and the growing tube eats its way down, as it were, feed- 

 ing upon the substance of the pistil as it grows. How is 

 it in the flower you are examining .•" In some orchids the 

 pollen tubes can be seen by the unaided eye, massed 

 together within the thickened style, looking like a strand 

 of fine white floss. It takes a grain of pollen to fertilize 



