ANGIOSPERMS. 583 



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. 391) 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 Plumbagineae, the conducting tissue of the style 

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

 micropyle ; and so forth. [The conducting tissue is also secretory, and it appears 

 that, as Amici originally suggested (and this view has been recently confirmed by 

 Dalmer), the pollen-tube obtains the materials necessary for its growth from the 

 secretion.] 



[We have already seen that two cells (at least) are formed in the pollen-grain of 

 Angiosperms, and that they are sometimes separated by a cellulose wall, but more 

 commonly by an ectoplasmic layer {hautschichi) of protoplasm. The pollen-tube is 

 formed from the larger of these two cells. It sometimes happens that the smaller 

 (vegetative) cell of the pollen-grain is unaffected by the formation of the tube, but 

 more commonly the layer separating the two cells is absorbed, and the two nuclei 

 travel, together with protoplasm, into the growing tube, the nucleus of the larger 

 cell frequently going first.] 



Since every ovule requires one pollen tube for its fertilisation, 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 vernus 

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

 from 5 to 10 cm. in length, those of 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 

 Orchideae 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 increasing 

 quickly in length ; after entering the micropyle its wall generally thickens rapidly and 

 often considerably, chiefly, as would seem, by swelling, so that its apical portion 

 communicates with the rest of it by only a narrow channel, or is entirely cut off. 

 Hofmeister compares it, in this condition, to a thermometer-tube (as e.g. in Lilium^ 



Elfving, Jenaische Zeitschrift, 1879, and Quart. Journ. Micr. Sci. XX, 1880. — Dalmer, Jen. Zeilsch. 

 1880.— Capus, Anat. du tissu conducteur, Ann. d. Sci. Nat. ser. 6. t. VII,] 



