302 



FERTILIZATION. 



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

 the winds for many miles. 



573. The pollen of Pines and other Gymnospermous plants falls 

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

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

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

 sider. 



574. The Action of Pollen on the Stigma. The loose papilla, 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 through 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 downwards into the style, gliding along the inter^'- 

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

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

 ovary, and at length reaches the placeiita, 

 or some other part of the lining off the 

 ovary, and its extremity appears in | the 

 cell. This prolongation into a tube, olften 

 many hundred times the diameter off the 

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

 ner of elongating cells (37 - 97), nouri.4ilied 

 by the organizable moisture of the si^yle 

 which it imbibes in its course. Now the 

 orifice of the ovules, or a projection ' of 

 the nucleus beyond the orifice, is at tljis 

 time brought into contact with, or cloi^e 

 proximity to, that portion of the walls of the ovary from which th^e 

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

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



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

 ai Convolvulus, with its tube. 539. Otber 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 opposite angle also emitting a 

 pollen-tube. 



