302 



FERTILIZATIOX. 



time is almost loaded with pollen, some of which is often wafted by 

 the winds for many miles. 



573. Tlie pollen of Pines and other Gymnospermous plants falls 

 directly upon the naked and exposed ovules (560). On all others, 

 the ovules, being secluded in a closed ovary, can be fertilized only 

 through tlie stigma. In these, accordingly, we have first to con- 

 sider. 



574. The Action of Pollen on the Stigma. The loose papillae, or 



often the short projecting hairs of the stigma, and the moist surface, 

 serve to retain the grains of pollen on the stigma when they have 

 once reached it. Absorbing some of this moisture, and nourished 

 by it, the grains of pollen which are favorably situated soon begin 

 to grow, or, as we may say, to germinate. The thin inner mem- 

 brane (534) extends, breaks thi'ough the thicker, but weak or brittle, 

 outer coat at some point (or rarely at two or three places), and 

 lengthens into a delicate tube, filled with the liquid and molecular 

 matter that the grain contains. This tube (Fig. 537 — 540), remain- 

 ing closed at the extremity, penetrates the loose tissue of the stigma, 

 and is prolonged downAvards into the style, gliding along the inter- 

 spaces between the very loosely disposed cells of the moist conduct- 

 ing tissue (541), which extends from the stigma to the cavity of the 



ovary, and at length reaches the placenta, 

 or some other part of the lining of the 

 ovary, and its extremity appears in the 

 cell. This prolongation into a tube, often 

 many hundred times the diameter of the 

 pollen-grain, is a true growth, after the man- 

 ner of elongating cells (37 - 97), nourished 

 by the organizable moisture of the style 

 Avhich it imbibes in its course. Now the 

 orifice of the ovules, or a projection of 

 the nucleus beyond the orifice, is at this 

 time bi'ought into contact with, or close 

 proximity to, that portion of the walls of the ovary from which the 

 pollen-tubes project ; and a pollen-tube thus enters the orifice of 

 each ovule, and reaches the nucleus, in which the nascent embryo 



537 



538 



539 



FIG. 537. A pollen-grain of Datura Stramonium, emitting its tube. 538. Pollen-grain of 

 a Convolvulus, with its tube. 539. Other pollen-grains, with their tubes, less strongly mag- 

 nified. 540. A pollen-grain of the Evening Primrose, resting on a portion of the stigma, into 

 which the tube emitted from one of the angles penetrates ; the oppoiite angle also emitting a 

 pollen-tube. 



