260 FORMATION AND STRUCTURE OF THE POLLEN. 
monly of a waxy nature, to which the name of pollinia has been 
given (fig. 564, p). In the Asclepiadacez somewhat similar 
masses occur (fig. 565, p, and b); but in the latter, the whole 
surface of each pollen-mass is invested by a special cellular 
covering. By a careful examination of these pollinia we find 
that they are formed of compound masses agglutinated together, 
and when separated, each of these masses is found to consist of 
four pollen-cells. In the pollinia of the Orchidaceze we also 
find other peculiarities ; thus each is prolonged downwards in 
the form of a stalk called the cauwdicle (fig. 564, c), which adheres 
commonly at the period of dehiscence to one or two little 
glandular masses called retinacula (figs. 566, a, and 564, 7, 7), 
Fic. 564, Fig. 565. Fic. 566. 
Fig. 564. Pollinia, p, of a species of Orchis with their caudicles, c, adhering 
to the retinacula, 7°, 7.— Jig. 565. Pistil of a species of Asclepias, with 
the pollinia, p, adhering to the stigma,s. 6. Pollen-masses separated. 
— Fig. 566. Upper part of the flower of an Orchis, showing the pollinia 
adhering to the stigma by the retinacula, a. 
which are placed on the upper surface of a little projection of 
the stigma or style, called the rostellwm, which is situated at the 
base of the anther. 
Structure of the Pollen.—We must now more particularly 
describe the structure of the pollen-cell, or pollen-grain as it is 
more frequently called. We shall treat of it under three heads, 
viz. :—1. Its Wall or Coats; 2. Its Contents ; and 3. Its Form 
and Size. 
1. Wall or Coats of the Pollen-Cell.—When mature the wall of 
the pollen-cell generally consists of two membranes : an internal 
or intine, and an external or extine. In rare cases the outer 
coat appears to consist of two, or even three layers ; while in 
Zostera, Zannichellia, and some other submersed aquatic plants, 
there is but one membrane, which is of a similar nature to the 
intine. 
