BOTANY OF THE LIVING PLANT 



to contain four sporangia, or pollen- sacs, two being placed on either 

 side of a central connective, which is simply a continuation upwards 

 of the filament. A single vascular strand which traverses the filament 

 is continued upwards into the anther, where it fades out usually 

 without any branching (Fig. 202, A). Stamens arise as exogenous 

 growths from the axis, and in acropetal succession. In " doubled " 

 (lowers they not infrequently appear as transformed into petals, or 

 even into green leaves. This suggests a foliar nature. But for a full 

 understanding of their relation to foliage leaves comparison will be 



Fig. 203. 



Lobe of anther of Caltha cut transversely, showing two pollen-sacs at maturity, 

 with the fibrous layer immediately below the epidermis. For details see text. 

 ( x 100.) F. O. B. 



needed with corresponding parts in early fossils. For close comparison 

 the material is gradually becoming available : but at the moment that re- 

 lation appears as an evolutionary problem rather than a demonstration. 

 It may, however, be accepted that stamens are parts specialised for 

 bearing pollen-sacs or micro -sporangia. Each pollen-sac is enclosed 

 till it is ripe by a wall consisting of several layers of cells. In most 

 stamens the slit of dehiscence runs longitudinally, following the line 

 where the walls of the two sacs of one anther-lobe join with the 

 septum that separates them (Fig. 201). There the cell-walls are thin 

 and the cells themselves are rounded off by intercellular spaces, so 

 that they easily come apart. The slit thus formed gapes widely, owing 

 to the action of the walls of the pollen-sacs. Below their superficial 

 epidermis lies a layer of fibrous cells, the inner cell- walls of which are 

 thicker than the outer, while fibrous bars running outwards along 

 their lateral walls prevent radial collapse. The effect on these cells of 



