232 Practical Plant Biology. 



with minute tubercles of cutinised cellulose. Each of the cells so 

 formed is a pollen-grain. While the pollen-grains are being de- 

 veloped the filament of the young stamen elongates, and changes 

 occur in the outer layers of the anther-lobes. The cells forming 

 the tapetum are absorbed, apparently being used up in the forma- 

 tion of the pollen-grains and their walls. The pollen-grains thus 

 come to lie loose in the four cavities the four pollen-sacs two in 

 each lobe of the anther. The superficial layers of the anther form 

 the outer walls of these pollen-sacs, and are composed of the 

 epidermis and the fibrous layer. The walls of the cells of this 

 layer are thin but strengthened by the deposit of fine ridges of 

 lignified cellulose on their inside. These ridges are absent from 

 the outer wall. They form reticulations on the inner wall, and 



FIG. 81. Ranunculus repens, anther, transverse section, x 70. a, connective ; 

 b, pollen-sacs; c, epidermis; d, fibrous layer; e, line of dehiscence ; 

 /, broken partition between pollen-sacs ; g, pollen-grains. 



they project out along the radial walls in a radial direction. It 

 will be understood that with this form of support the outer walls 

 of the fibrous layer are free, with the loss of water in ripening, to 

 crumple, while the inner walls remain rigid. This leads to a strain, 

 and to a tendency to a change in curvature of the wall of the 

 pollen-sac which is relieved finally by the tearing of the wall. The 

 place where the rupture will occur is decided by the weak coherence 

 of the cells of the epidermis along a band just opposite the parti- 

 tion which separates the two pollen-sacs of the anther. The split 

 in the wall along this line and the recurving of its parts simultan- 

 eously break into both of its sacs and set free the pollen. 



Above the tapering cone on which the stamens are set the axis 

 expands into a small rounded knob. On this are set a number of 

 carpels. The carpels have the form of flasks flattened below on 



