OF THE POLLINIUM IN ASCLEPIAS CORNUTI. 81 
where the preparation has been treated with alcohol, slightly contracted away from this 
wall. The membrane, however, is exceedingly difficult to detect at this stage. This 
change takes place simultaneously in all the special mother cells. The newly formed cell, 
consisting of a very thin and delicate cellulose wall, closely applied to the internal side of 
the pale-yellow cuticularized wall of the special mother cell by which it is surrounded, 
but from which it may be made to contract away by means of alcohol, enclosing proto- 
plasm loaded with vacuoles and rendered dark with minute granules and a nucleus, is 
the equivalent of the pollen-grain of other plants, and, to indicate this feature, is here 
designated by the same title. 
The mode of formation of the pollen, then, in Asclepias is very different from that which 
is the characteristic and prevalent type in the majority of Dicotyledons or Monocotyledons, 
and, so far as our present knowledge extends, exhibits in its entire details a perfectly 
unique, isolated, and peculiar case of development. The earlier stages are only to be 
found paralleled in the single instance of Zostera, which affords either the most primitive 
or most aberrant type of pollen-formation known. The later stages find no precise 
parallel in the entire range of the vegetable kingdom. This is the more remarkable, 
since another member of the Asclepiades, viz. Periploca greca, exhibits, according 
to Reichenbach, a type of pollen-formation exactly comparable to that of the Orchid 
genera Neottia and Epipactis *. 
Observations on the mode of development of the pollen in Asclepias are fraught with 
extreme diffieulty ; and its history ean only be revealed by careful study of extremely 
thin transverse and longitudinal sections. 
In many of the pollen-grains, especially when the flower was fully mature, I was able, 
by careful observation, and by having recourse to osmic acid of one per cent. strength, 
and to staining reagents—such, e. g., as heematoxylon, Grenicher's carmine, and some of 
the aniline colours, viz. gentian-violet, saffranin, and methyl-green, tothe latter of which 
а few drops of solution of acetic acid, one per cent. strength, had been previously added 
—to detect not a single nucleus only, but two nuclei, one of which was invariably larger 
than the other. | 
The smaller nucleus was often found lying close to Ше cell-wall; and in these cases I 
believe that, surrounded by a small quantity of protoplasm, it is cut off from the rest of 
the grain by a cellulose wall, although I was not always able to show this satisfactorily. 
This discovery is especially of interest in connexion with the recent researches of 
Elfving f and Strasburger 1, since further confirmation of their observations has thereby 
been obtained in the pollen-grains of plants which they did not investigate, and in 
* Dr. S. H. Vines suggests that probably in Asclepias and likewise in Zostera the phase of the special mother 
cells, as it oceurs in other plants, is omitted, and hence we get the departure from the normal types. On this view, 
what I have termed the special mother cells are really the last series of the mother cells produced by repeated 
division of the single primary one. 
+ Jenaische Zeitschrift für Naturwissenschaft, 1879, part i., and Quarterly Journal of Microscopical Science, М. $, 
vol, хх. 1880, рр. 19-85. 
i * Ueber Befruchtung und Zelltheilung," Jenaische Zeitschrift für Naturwissenschaft, Bd. xi.; (neue Folge, 
Bd. iv.) 1877, Heft iv. page 450. 
