236 



COMPENDIUM OF GENERAL BOTANY. 



or endothecmm. It is supplied on its inner Avail and on the radial 

 walls, as seen in cross-section, with tliickened bands which project , 

 inward. The outer wall in contact with the epidermis does not 

 contain these thickenings. These bands extend at right angles to 

 the epidermis on the radial walls, those of the inner walls extend 

 in all directions. The result of this thickening is that on drying 

 the radial walls contract much more rapidly in the tangential direc- 

 tion than do the inner walls. The thickenings in the radial walls act 

 as levers, exerting a force outwardly, but not inwardly. The ten- 

 dency to shorten, which is manifest in the, radial walls, is neutral- 

 ized as soon as the margins of the valves become separated from 

 the i3illar of the connective. The opening and recurving of the 

 valves is a very sudden explosive act, whereby the pollen-grains are 

 thrown out with considerable force. In the sporangium of ferns 

 the weakest point corresponds to the group of tliin-walled cells 

 terminating the annulus. The latter is also the outermost cell- 

 layer (see Fig. 128, C). The radial 

 walls of the annulus lying parallel to 

 tlie plane of the ring, as well as the 

 outer walls, are thin ; the remaining 

 radial Avails and the inner walls aye 

 thick. The cause for the immediate 

 opening of the annulus (sporangium) is 

 the same as in the anther, only that in 

 the sporangium the evaporation of moist- 

 ure is very rapid, since it can jjass at 

 once to the atmosphere through the thin 

 outer walls of the cells.' 



Keturning to the structure of an- 

 thers, we must not forget to mention 

 that in anthers which open by pores 

 the fibrous layer is entirely absent, since 

 they do not require any mechanism 

 for opening {Ericacece, Pirolacece, 

 MelastomacecB) . 



The pollen-grains are isolated cells or small cell-bodies of 

 spherical or oval form, with a dotible cell -membrane, the intine and 



Fig. 154. — Pollen-grain of 

 lobium, angustifolium. 

 (After Sachs.) 



• Among others Mohl, Chatin, Schinz, Prantl, and Schrodt made special 

 studies of this subject. 



