3 68 



FOURTH GROUP. SEED-PLANTS. 



its thickness in a radial direction (Fig. 297), and making it appear to be composed 

 of rod-like prismatic pieces or honey-comb-like lamellae or the like, structural con- 

 ditions which recall the episporium of the Marsiliaceae. The contents of the ripe 

 pollen-grain, the fovilla of the older botanists, usually consists of dense coarsely- 

 granular protoplasm, in which grains of starch and small drops of oil may be detected ; 

 if the grain is ruptured in water, the fovilla appears in masses of a coherent mucilage 

 which often form long vermiform convolutions. Oil of a yellow or of some other 

 colour is often found on the outer surface of the exine, often in perceptible drops, 

 which makes the pollen sticky and adapted for conveyance by insects from flower to 

 flower ; in a few plants only it is dry and powdery, as in the Urticaceae and many of 

 the Gramineae, where it is flung violently from the anthers or simply drops from them. 

 As the pollen-grains approach maturity and the flower-bud is preparing to open, 

 the wall of the loculament also undergoes further change 1 . The walls of the outer layer 

 of cells, the epidermis (exothecium of authors), are always smooth (Fig. 299 G, H], and 

 those of the inner layer or layers (the endothecium of authors 2 ) are also smooth, if the 

 anther does not dehisce; but if valves are formed (Fig. 274 K), the cells of the inner 

 layers are furnished with thickening-bands on the valves only, that is are fibrous, while 



if the loculaments dehisce longitudinally, 

 all parts of the endothecium contain 

 fibrous cells ; there is usually only one layer 

 of this kind, sometimes several, in Agave 

 americana as many as from eight to twelve. 

 The thickening-bands of the fibrous cells, 

 which project inwards, are usually wanting 

 on the outer wall ; on the lateral walls they 

 are usually perpendicular to the surface of 

 the loculament, and on the inner wall 

 of the cells they run transversely and are 

 connected in a reticulate or stellate manner. 

 When the walls of the mature anthers 

 dry up, the epidermal cells contract 

 more strongly than the cells of the 

 endothecium which are provided with the 

 thickening-bands, and consequently exert a force which tends to make the wall of the 

 anther concave outwards and to rupture it at the weakest spot. The modes in which 

 the pollen-sacs dehisce are very various, and have a close and constant relation to the 

 rest of the arrangements for pollination in the flower, whether by insects or by some 

 other means. Sometimes a short fissure only is formed at the apex of each anther- 

 lobe, as in Solatium and the Ericaceae (Fig. 275), by which the pollen escapes from 

 both the adjoining loculaments, or, and this is the commonest case, the wall parts 



FIG. 298. Pollen-tetrads of Neottia Nidus avis, one of the 

 Orchideae. The disposition of the four daughter-cells varies 

 much according to the shape of the pollen-mother-cell before 

 division. The contents are shown in A in one of the right hand 

 cells, which contains two nuclei, one belonging to the ' vege- 

 tative' cell. 



1 H. v. Mohl, Verm. Schriften, p. 62. Purkinje, De cellulis antheramm fibrosis, Vratislaviae, 

 1830. 



2 [If Purkinje's terms exothecium and endothecium are to be retained (and they might be 

 dropped with advantage), they should only be used in describing the state of the anther-wall at the 

 period of maturity and dehiscence and without reference to development, exothecium being the 

 epidermal layer (a single one), endothecium all the layers visible beneath it.] 



