ANGIOSPERMS. ^09 



function ; the morphological significance of these parts is still undecided ; and it 

 remains at present doubtful whether the two embryonic vesicles correspond to 

 those of Santalum and Watsonia ; or whether the one which is destined to destruc- 

 tion must not rather be considered as the separated canal-cell, the other as the true 

 oosphere. 



In a few cases polyembryony occurs also among Angiosperms ; but it is 

 brought about in a way different from what occurs in Gymnosperms. A number 

 of embryonic vesicles are formed in the parietal protoplasm of the embryo-sac 

 before impregnation in Funkia cceruha, Scabiosa (according to Hofmeister) and 

 Citrus ; the formation of embryos is caused in them by the contact of the pollen- 

 tube with the apex of the embryo-sac ; but of the large number of rudimentary 

 embryos, which is very considerable, especially in Citrus, only a few attain to a 

 capacity for germination. 



Fertilisation^. The pollen-grains which germinate on the stigma send out 

 their tubes through the channel of the style where there is one, or more usually 

 through the loose conducting tissue in its interior, down to the cavity of the ovary ; 

 frequently both in erect basilar (Fig. 361) and in pendulous anatropous ovules the 

 micropyle lies so close to the base of the style that the descending pollen-tube 

 can enter it at once. But more often the pollen-tubes have to undergo further 

 growth after their entrance into the cavity of the ovary before they reach the micro- 

 pyles of the ovules ; and they are then guided in the right direction by various 

 contrivances. We frequently find papillose projections of the placentae or other 

 parts of the wall of the ovary, to which the pollen-tubes attach themselves ; in our 

 species of Euphorbia a tuft of hairs conducts them from the base of the style to 

 the neighbouring micropyle ; in the Plumbaginese, the conducting tissue of the style 

 forms a conical descending outgrowth, which conducts the pollen-tube into the 

 micropyle ; and so forth. 



Since every ovule requires one pollen-tube for its impregnation, the number of 

 tubes which enter the ovary depends, speaking generally, on the number of the 

 ovules contained in it ; the number of pollen-tubes is however usually larger than 

 that of the ovules; where these latter are very numerous, the number of pollen- 

 tubes is therefore also very large, as in Orchideae, where they may be detected in 

 the ovary even by the naked eye as a shining white silky bundle. 



The time that intervenes between pollination and the entrance of the pollen- 

 tube into the micropyle depends not only on the length of the style, which is often 

 very considerable (as in Zea and Crocus), but also on the specific characters of the 

 plants. Thus, according to Hofmeister, while the pollen-tubes of Crocus vermis 

 only require from twenty-four to seventy-two hours to penetrate the style which is 

 from 5 to 10 cm. in length, those o( Arum maculatum take at least five days, although 

 the distance they have to go over is scarcely more than 2 or 3 mm., and those of 

 Orchide^ require ten days or even several weeks or months, during which time the 

 ovules first become developed in the ovary, or even are not formed till then. 



The pollen-tube is usually very slender and thin-walled as long as it is increas- 



' Besides the works of Hofmeister already quoted, see his historical account in Flora, 1857, 

 p. 125, where the literature is collected. 



