55^ PHANEROGAMS. 



Malvaceae, Alsinese, &c. (see Schacht, /. c). The extine is rarely smooth, more often 

 marked on the outside by the sculpture to which reference has already been made. 

 When it is very thick, layers of different structure and texture may frequently be 

 detected, and differentiations sometimes occur in a radial direction, penetrating the 

 thickness of the extine (Fig. 381), and giving it in some cases the appearance of con- 

 sisting of rod-shaped prismatic pieces or of honeycomb-like lamellae, &c., peculiarities 

 of structure recalling those of the epispore of Marsiliaceae. The contents of the ripe 

 pollen-grain, the Fovilla'^ of the older botanists, usually consists of a dense coarse- 

 grained protoplasm in which grains of starch and drops of oil may be recognised. 

 When the grain bursts in water, the fovilla escapes in masses connected by mucilage 

 and often in long vermiform threads. The surface of the extine is commonly found 

 coated with a yellow oil, or of some other colour, often in evident drops, which 

 renders the pollen viscid and adapted to be carried by insects from flower to flower ; 

 in only a comparatively few cases is it quite dry and powdery, as in Urticaceae 

 and many Grasses, where it is projected with violence from the anthers or simply 

 falls out. 



At the time when the pollen-grains are nearly mature, and the flower-bud is 

 preparing to open, the wall of the pollen-sacs undergoes a further development ^. 

 The outer layer of cells or epidermis always remains smooth-walled (see Fig. 382, 

 P- 558) ; th^ inner layers or endothecium are also smooth if the anther does not 

 dehisce. If on the other hand it opens by recurved valves (Fig. 362 U), the cells of 

 the inner layers of these valves only are provided with thickening-bands (or are 

 fibrous) ; while, when the pollen-sacs dehisce longitudinally, the whole of their endo- 

 thecium contains fibrous cells. There is usually only one such layer, 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 their outer wall; on 

 the side walls they are generally vertical to the surface of the pollen-sac; on the inner 

 wall they run transversely and are united in a reticulate or stellate manner. Since the 

 epidermal cells contract more strongly when the ripe anther-walls dry up than those 

 of the endothecium which are provided with thickening-bands, they exert a force 

 which has a tendency to make the anther-wall concave externally and to give way at 

 its weakest point. The modes in which the anthers open are very various, and are 

 always intimately connected with the other contrivances which are met with in the 

 flower for the purpose of pollination with or without the agency of insects. Sometimes 

 only a short fissure (pore) is formed at the apex of each anther-lobe, as in Solanum^ 

 EricacecE (Fig. 363), &c., through which the pollen of both the contiguous pollen-sacs 

 escapes ; but more commonly the wall gives way in the furrow between the two sacs 

 (the suture) along its whole length, the tissue which separates them becoming at the 

 same time more or less destroyed, and thus both pollen-sacs dehisce at the same 

 time by the longitudinal fissure (Fig. 382). It is this phenomenon that has given rise 

 to the erroneous description of these anthers as being bilocular ; but if nomenclature 

 is to have a scientific basis, they must be termed quadrilocular, in contrast to the 



^ [On the constitution of the ' amyloid corpuscles ' in the fovilla of pollen see Saccardo, Nuovo 

 Giornale Botanico Italiano, 1872, p. 241.] 



^ Compare H. v. Mohl, Vermischte Schriften, p. 62. — Purl<yne, De cellulis antherarum fibrosis, 

 Vratis. 1830. 



