CHALAZOGAMIC FERTILIZATION. 



413 



not so direct. Here it passes by tlie embryo-sac on one side and then turns sharply 

 down again, reaching the egg-cell from the same direction as it would have done 

 had it entered by the micropyle (c/. fig. 314 B, p.t.). It is interesting to note that 

 in many nearly allied Amentacese, as in the Cupuliferaj, which includes the Oak 

 (Quercus), Beech (Fagus), and Chestnut (Castanea), fertilization is by the micropyle. 

 The following table indicates the method of fertilization as at present known in 

 the various families comprehended in the group 

 Amentacese: — ■ 



Fig. 314 B.— Chalazogamic fertilization in the 

 Alder (Alnus glutinosa). Diagrammatic. 



The drawing shows one half of a longitudinal 

 section of the ovary. The wall of the ovary 

 is thick, and has a hardened middle layer 

 (shaded dark)- At the top is the base of the 

 style, from which the pollen-tube {p.t.) can 

 be traced passing straight on through the 

 substance of the ovary to the ovule. Enter- 

 ing the ovule by its point of attachment to 

 the placenta the pollen-tube bends sharply 

 upwards (at the chalaza) into the nucellus. 

 It now passes by the small oval embryo-sac 

 on the inner side, and when below the micro- 

 pyle turns sharply down to the apex of the 

 embryo-sac. The vascular supply of the 

 ovules, which forms a column in the placen- 

 ta, is shaded dark. Considerably enlarged 

 (from a drawing by M. F. Ewart). 



Many experiments have been made from 

 time to time with a view to explaining the 

 phenomena attendant on the wandering of the 

 pollen-tube from the stigma to the micropyle. It 

 has been shown that the pollen-tube is extremely 

 sensitive to various external conditions, and that 

 by appropriately vaiying these the direction 



followed by the tube in its growth may be controlled. Pollen-tubes are especially 

 sensitive towards sugar-solutions, and bend out of their course towards the sugar. 

 They also tend to grow away from the air, and show a preference for spaces satu- 

 rated with aqueous vapour to such as are less humid. Of all the conditions which 

 affect a pollen-tube, most conspicuous is the attraction which sugar exerts upon it. 

 Various portions of the pistil exert a similar chemical stimulus on pollen-tubes, very 

 marked being the action of the micropyle in this respect. A few of the experiments 

 demonstrating this attractive property of stigma and ovules may be briefly 

 described. If a fresh mature stigma be cut oft' and laid on a plate of gelatine, and 



