156 VEGETABLE PHYSIOLOGY. 



mode of transport : in some the pollen is powdery and abundant, as in Pines, Spinach, 

 &c. ; tlie pendent anthers of others at the least breath scatter the pollen ; in others 

 the perianth is wanting, or the stigmas project beyond the flower at the moment of 

 fertilization ; in some the flowers appear before the leaves ; and some have feathery 

 stigmas, as Graminece> Mercurialis, &c. Wind-fertilized flowers do not secrete 

 nectar ; the pollen is too dry to adhere to insects, and the corolla is either absent, or 

 possesses neither the colour, scent, nor nectar which attract them. 



We shall conclude these remarks by mentioning the curious phenomena re- 

 specting the fertilization of Vallisneria spiralis, which grows submerged in stagnant 

 waters in the south of France. It is dioecious, but the male plants always grow near 

 the female ; the female flower, protected by a spathe, is borne on a long peduncle 

 which rises from a tuft of radical leaves ; and the ovary bears three forked 

 stigmas. The male flowers are borne on a very short peduncle, and are sessile on 

 a conical axis enveloped in a spathe. At the flowering period the female peduncle 

 gradually lengthens, so that the flower finally floats on the surface of the water, 

 and opens its perianth of six very minute segments. Then the male flowers, which 

 have hitherto remained submerged, detach themselves spontaneously from their 

 peduncle, and rise to the surface, where numbers of them may be seen floating 

 around the female flower, on which the anthers elastically project an abundance of 

 pollen. After fertilization, the peduncle of the female flower contracts spirally, and 

 the ovary descends to the bottom of the water to ripen its seeds. 



In describing the anther, we spoke or the fibrous cells which, after the 

 maturing of the pollen, form a layer upon the inner wall ; which layer gets thinner 

 as it approaches the line of dehiscence, where it disappears. At the moment when 

 the pollen is ready to be discharged, the moisture of the anther evaporates, its 

 hygrometrical tissue, pulled different ways by the variations of the atmosphere, 

 produces a strain along the line where the fibrous cells are interrupted, and these 

 by their contraction favour the emission of the pollen. At the same time the cells 

 of the stigma become viscous, so as to retain the pollen projected on to them from 

 the anther, or carried thither by the wind or by insects. Thereupon the pollen 

 swells, through the action of endosmose ; the inner membrane ruptures the outer at 

 one of the points which touch the stigma; the pollen-tube (fig. 413) lengthens, 

 traverses the interstices of the stigmatic cells, and reaches the conducting tissue 

 which fills the canal of the style, and which is charged, like the stigma, with a 

 thick fluid. Still lengthening, the pollen-tube finally enters the cavity of the 

 ovary, traverses the conducting tissue which lines the placentas, and at last 

 reaches the ovule (fig. 750), when it enters the micropyle and comes in contact 

 with the cell of the nucleus (embryonic sac), its tip resting on the membrane of the 

 sac, and partly adhering to it. Soon after this contact of the pollen-tube, one, 

 or oftener two vesicles (embryonic vesicles, fig. 750) usually appear within the 

 embryonic sac, below the tip of the pollen-tube. These vesicles elongate ; the 

 upper and thinner end adhering to the membrane of the sac. While one of the two 

 shrinks and disappears, the other develops, and fills more or less completely with 

 its free end the cavity of the embryonic sac. The embryonic vesicle, which will 



