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 Marsihacea^. 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 j 
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 2. 
The outer layer of cells or epidermis always remains smooth-walled (see Fig. 382, 
P- 558) ; the 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 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. SomxCtimes 
only a short fissure (pore) is formed at the apex of each anther-lobe, as in Solanum^ 
Ericacecß (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. — Purkyne, De cellulis antherarum fibrosis, 
Vratis. 1830. 
