502 



BOTANY 



PART II 



exit is, however, prevented by the liairs until the stigma has withered and 

 the anthers have shed their pollen. When this has taken jilace (Fig. 457 //) the 

 hairs dry up, and the insects covered with pollen can make their way out and 

 convey the pollen to the receptive stigmas of younger flowers. In the Orchidaceae 

 and Asclepiadaceae (Fig. 686) self-pollination is rendered impossible both by the 

 nature of the pollen- masses and by their position. A complicated form of structural 

 contrivance, by means of which cross-pollination is secured, may be seen in a 

 ^ovi ex o^ Salvia pratensis (Fig. 458). The anthers of this flower are concealed in 

 the upper lip of the corolla, from which the style, with its bilobed stigma, projects. 

 When the humble-bee visits the flower in search of honey, it must first with its 

 proboscis push out of the way the small plate (s), formed of two sterile anther 

 halves grown together. These are situated at the ends of the short arms of the 

 connectives (c), which are so elongated that they might easily be mistaken for the 



Fig. 458.— Pollination of Salvia ivatcnsU. 1, Flower visited by a humble-bee, sliowing the projec- 

 tion of the curved connective from the helmet-shaped upper lip, and the deposition of the 

 pollen on the back of the humble-bee ; 2, older flower, with connective drawn back, and 

 elongated style ; 4, the staininal apparatus at rest, with connective enclosed within the upper 

 lip ; 3, Uie same, when disturbed by the entrance of the proboscis of the bee in the direction of 

 the arrow ; /, filament ; c, connective ; s, the obstructing half of the anther. 



filaments (/) of the stamens. The fertile anther-halves are situated at the other 

 ends of the connectives, and are thus brought in contact with the hairy back of 

 the humble-bee when it pushes against the plate at the short ends of the lever-like 

 connectives. The pollen thus attached to the bee will be brushed off its back by 

 the forked stigma borne on the elongated style of an older flower (Fig. 458, 2). 



Development of the Sexual Generation in the Phanerogams 



Having become acquainted with the various adaptations by which 

 the union of the sexual products, in the Phanerogams, is made possible, 

 we have to follow the course of development in the macrospores and 

 microspores leading to the formation of the sexual products and the 

 method by which the union of these is effected. 



In the Gymnosperms (**) a prothallium consisting of a few cells is formed on 

 the germination of the microspore. This lies within the large cell, which will 

 later give rise to the pollen-tube, closely applied to the cell wall. The protlialiium 

 is composed of from 1 to 3 cells. The first-formed cells (Fig. 459 A-C, c) correspond 

 to the vegetative cells of the prothallium. The spermatogenous cell {sp), which is 

 cut off after these, divides later into the mother-cell of the antheridium and a 



