ANGIOSPERMS. 
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 Plumbaginese, 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 {hautschicht) 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 
Orchidese 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, Quart. Journ. Micr. Sei. XX, 1880. — Dalmer, Jen. Zeitsch, 
1880. — Capus, Anat. du tissu conducteur, Ann. d. Sei, Nat. ser. 6. t. VII.] 
