THE AXGIOSPERMAE 1197 



the same as in the latter plant, namely by collapse of the cells follow- 

 ing the rapid extrusion of vacuolar sap. In Berberis the motor-tissue 

 is limited to the base of the filament on the adaxial side, whereas in Cen- 

 taurea it surrounds the base of the filament. In the former the movement 

 is therefore an adaxial curvature, while in the latter it is one of simple 

 contraction. 



The sensitive region in the stamen of Berberis, as in a number of other 

 examples (Opiintia, Portiilaca, Catasetiim), differs from the rest of the fila- 

 ment surface in that the epidermal cells protrude as prominent papillae 

 which are the receptors of the touch stimulus. In Centaurea, on the other 

 hand, the sensitive stamens are surrounded by a belt of bristly hairs, partly 

 united into a collar, which in this case are the touch-receptors. In Sparmannia 

 the receptors are downwardly curved folds of the epidermis. All alike react 

 to the contact of an insect's proboscis when probing the flower for nectar. 



The anther begins its development as a homogeneous mass of paren- 

 chyma, in the median line of which runs a strand of desmogen that differen- 

 tiates at an early stage into the vascular bundle of the connective. The 

 outline then becomes, first, bilobed and secondly quadrilobed, each lobe 

 being one microsporangium. In the tissue of the lobe certain cells of the 

 sub-epidermal layer begin to enlarge, their walls becoming thicker and their 

 protoplasm denser than in the neighbouring cells. These are the initial 

 cells of the archesporium. The number of these cells is very variable. 

 In some species of Mentha they form a layer almost all round each lobe of 

 the anther. At the other extreme, in Malvaceae and Compositae, there is 

 only one cell in transverse section and in some Mimosaceae with small 

 anthers only one cell altogether. Generallv thev form a vertical file of 

 cells running almost the whole length of the lobe, but in certain cases, where 

 the mature pollen-sacs are not continuous but are transversely septate 

 {e.g., Onagraceae and other examples cited previously), the file of cells is 

 interrupted by unchanged parenchyma cells. 



The first division of the initial cells is periclinal, separating the primary 

 parietal cells on the outside from the primary sporogenous cells on 

 the inside. The parietal cells usually undergo further periclinal divisions, 

 forming several concentric layers which constitute the endothecium of 

 the anther wall. 



The archesporial tissue develops by the division of the sporogenous cells, 

 in two principal ways. The commoner method is by division in two direc- 

 tions, so that a solid mass of archesporium results, but in a minority of cases 

 {e.g., Malva, Mentha, etc.) the divisions are radial only and the arche- 

 sporium forms a single-layered arc of cells. The subsequent growth of the 

 archesporium cells is accompanied by the dissolution of the middle lamellae 

 between them. At this point in development the growth of the arche- 

 sporium lags behind that of the outer tissues of the anther, so that contact 

 between the two zones is broken. In the enlarged internal space, now a 

 pollen -sac, the individual cells of the archesporium fall apart and round 

 themselves off, becoming thus the pollen mother cells, in which meiosis 



