POLLINATION 169 



genera in which a typical fibrous layer is absent, and the sacs open by 

 longitudinal breaks — Orchidaceae, Araceae, Asclepiadaceae, Oroban- 

 chaceae, Diospyros. The anther wall is described as consisting of the 

 fibrous layer only in Althaea, Vitis, PhijfeJephas. (The epidermis has 

 been reported ephemeral in Vitis. ) The Orchidaceae and Orobanchaceae, 

 considered to lack a typical fibrous layer, have some wall cells with 

 sparse or incomplete bars; these cells probably represent vestiges of an 

 ancestral fibrous layer. Nonfunctioning fibrous cells are also present in 

 parts of the anther sac that do not open — Berberis. 



The shape of the cells of the fibrous layer and the position of the 

 thickened bars are doubtless related to the functioning of the layer, 

 though there seems to be no consistency in these features. The long 

 axis of the cells is described as parallel to the slit in many genera — 

 SiJene, Erodiiim, Plantago, Commelina; and as at right angles to the 

 slit in many other genera — Pijnis, Tiarella, Lychnis, Geranium. Shape 

 of the cells appears unimportant, since it differs in related genera — Silene 

 and Lychnis; Erodiiim and Geranium. The thickening bars usually 

 extend at right angles to the slit; only rarely are they parallel to the 

 slit — Salvia. 



Any function in dehiscence of the "third layer," that below the 

 fibrous layer, is probably indirect or accessory only. The small, rather 

 loosely packed parenchyma cells of this layer commonly disintegrate just 

 before deliiscence, freeing the outer layers from attachment to the con- 

 nective below; this disintegration may aid indirectly in the contraction 

 of the external tissues. 



The role of the fibrous layer has perhaps been exaggerated, though 

 it is surely the important layer in this function. Apparently both this 

 layer and the epidermis can function alone, but, in most families, the 

 mechanism responsible for deliiscence seems to involve two, or all 

 three, of the layers. 



Dehiscence of the anther is similar to that of the carpel in that it 

 involves a sporophyll wall. 



POLLINATION 



It was long assumed that pollination in the angiosperms was primi- 

 tively by wind, anemophily; that pollination by water, hydrophily, and 

 by animals, entomophily and ornithophily, were derived methods. This 

 conclusion was based largely on the presence of anemophily in the 

 conifers and in simple, and therefore supposedly primitive, flowers, 

 especially those of the Amentiferae. The opinion that the primitive 

 method of pollination was by insects — though argued at the beginning 

 of the twentieth century — received strong support only twenty to forty 



