94 SXAMENS. 



side-walls. As the cells dry a contraction of the rod-like thickenings supervenes, 

 leading to a movement like that of the afore-mentioned hand when the tips of 

 the fingers approach one another. Simultaneously the thin outer walls are thrown 

 into folds, so that where a number of these cells are present, side by side, the 

 whole outer surface will contract. These cells, being appropriately distributed 

 over the wall of the anther, will cause the slit-margins to fold back or the valves 

 to be raised. Besides these, other forms of contractile cells are present, difiering 

 from those described chiefly in form rather than in their mode of action. 



It must suffice here to mention only a very few instances. The anther- wall in 

 Conifers consists of a single layer of contractile cells, whilst that of Agave reaches 

 the other extreme, there being six to eight layers of such cells present. As a rule 

 the contractile layer is covered externally by a layer of delicate, thin- walled cells, 

 known as the Exothecium; the contractile layer constitutes the Endothecium. 

 The lining of the pollen-chambers consists of yet a third layer, the tapetal cells. In 

 anthers which have dehisced this last-mentioned layer is rarely demonstrable, it 

 having been already absorbed. Of the various layers it is the middle one, the 

 endothecium (contractile cells), which is active in the various movements under 

 discussion. 



In the discharge of the pollen from the opened anthers a great variety of 

 methods prevails. In the Nettle and Mulberry the filament of the stamen uncoils 

 like a spring at the moment of dehiscence of the anther, and the pollen is forcibly 

 scattered (fig. 229). The whole event is instantaneous, and to the observer 

 resembles an explosion. In other plants dehiscence is accomplished quietly, and 

 the pollen, which escapes slowly, may be first of all stored up temporarily at 

 definite spots within the limits of the flower. This storage occurs a good deal 

 more frequently than is generally supposed, and stands in relation to various 

 events which will be fully discussed later on. In Papilionacese the liberated 

 pollen is deposited in the hollowed apex of the Keel; in the Violet it is stored 

 in the grooves of the lowest, spurred petal; in the Poppies, Roses, and Buttercups, 

 it falls, at any rate in part, on to saucer-like depressions of the petals. The dust- 

 like pollen as it falls from the anthers of the catkins of the Walnut, Hazel, 

 Birch, and Alder, is received temporarily on the upwardly-directed under-surfaces 

 of the flowers standing below (c/. fig. on p. 742, vol. i.). In Composites, Cam- 

 panulas, and several Stellatse, the pollen is stored on the style or stigma, but not, 

 as was previously supposed, upon the receptive portions of this organ. On the 

 contrary, it is retained here by various hairs and papillae, specially designed for 

 the purpose. Then, in the Proteacess again, the pollen is deposited, whilst the 

 flower is still in bud, upon the summit of the stigma, without, however, coming 

 into contact with the receptive spot; the stigma in this case serves, at the 

 commencement of flowering, as a temporary dep6t for the pollen. In Sarracenia 

 the pollen falls upon the stigma, which has the form of an expanded umbrella, 

 and here for a while it remains, but not in contact with the receptive points. We 

 shall hardly overstep the mark in saying that in some 20,000 species of plants 



