-408 FEBTILIZATION AND FORMATION OF FRUIT IN PHANEROGAMS. 



-out of the delicate inner coat of the grain in the form of a tube through the thin 

 3)laces in the extine. The structure and distribution of these thin spots has been 

 -already described (p. 102); it need only be added that a tube may be pushed out 

 at each or any of them. When pollen is artificially cultivated in a prepared sugar- 

 solution several tubes arise simultaneously from different spots, but, in nature, on 

 the stigma, the production of a single tube is the rule. The tube which contains the 

 whole of the contents of the pollen-grain (spermatoplasm) forsakes the extine, which 

 remains behind as a dead shell. Very soon after its appearance through one of 

 these holes in the extine, the pollen-tube comes to have a considerable diameter, 

 •often approaching that of the grain in size. The tube now elongates, growing always 

 ■at the expense of the stigma. Its mode of growth is similar to that of a fungal 

 hypha, and its relation to the stigmatic tissues resembles that of the hypha of a 

 parasitic fungus to its host. Like the parasite, it is able to penetrate the subjacent 

 tissue and to make its way through it for long distances. 



This penetration by the pollen-tube is certainly amongst the most remarkable 

 properties of flowering plants. The object of these wanderings is to reach and 

 fertilize the ovules contained — in Angiosperms — in the closed chamber of the ovary. 

 Whether the stigma be sessile upon the ovary or situated upon a style, the distance 

 to be traversed is considerable, and, in a very large number of cases, the way leads 

 through closed tissues. As the pollen-tubes travel as a rule by definite rows of cells 

 or tracks, we may assume that these latter are in some way specialized for their 

 conduction; still it is very puzzling to understand exactly in what manner these 

 cells become thus qualified. In all likelihood the pollen-tubes are attracted by 

 certain substances secreted by the tissues, which they have to traverse in order to 

 reach the ovules. Of these sugar seems to be the most important, and by a continuous 

 secretion of this (and possibly other substances), the tubes are led on to the ovules. 

 Casual allusion has already been made to the fact that the motile spermatozoids of 

 Cryptogams swim through the water to the archegonia (amphigonia) in response to 

 a somewhat similar stimulus (p. 68). 



Investigations into the course followed by the pollen-tubes in passing from the 

 .stigma to the ovules show that it varies in different cases-. Simplest, and perhaps 

 typical of what was formerly supposed to be the route universally followed, is the 

 •case of the Martagon Lily (Lilium Martagon, of. fig. 313 1). If the columnar style 

 of this plant be cut longitudinally one sees that it is penetrated by a canal which 

 narrows below towards the ovary, but widens out into a funnel at the stigma, where 

 it opens by a tri-radiate slit. The lips of this aperture bear numerous papiUae; to 

 these the pollen-grains become attached and here commence to form their tubes. 

 The tips of the pollen-tubes curve down into the funnel and grow along the cells 

 which line the style-canal (fig. 313 1). Passing down this canal, which is at this 

 time more or less mucilaginous, the poUen-tubes are led ultimately to the cavity of 

 the ovary in which are contained the ovules. 



Very different is the mode of travelling of the tubes in Grasses, of which Avena 

 datior (fig. 313 2) may be taken as type. Upon the spherical ovary of this plant 



