OF THE POLLINIUM IN ASCLEPIAS CORNUTI. 83 
tapetal membrane, together with a large portion of the substance of the anther, is 
broken down and disintegrated. "The comparatively late period to which the tapetal 
membrane persists in Asclepias is a noteworthy point; in other Dicotyledons it usually 
breaks down, in consequence of the growth of the pollen-grains, immediately after the 
absorption of the walls of their special mother cells, while in the group of Monocoty- 
ledons it becomes either diffluent or absorbed at an early period, and the mother cells 
themselves in consequence float freely about in the loculus quite separate from one 
another. 
Asclepias therefore appears to present at first sight a closer analogy in the period of 
resolution of its tapetum to the Monocotyledons than to the group of which it is a 
member, since the pollinium, which consists, among other parts, of the persistent though 
altered walls of the mother cells, comes ultimately to lie in the cavity formed by its 
resolution. Inasmuch, however, as the period of its resolution is coincident with that 
of the dehiscence of the anther-loculus, I believe that it more closely approaches the 
type of the group to which it really belongs than that of the Monocotyledons, though 
it differs from both, so far as we know of them at present. 
DESCRIPTION OF THE PLATE. 
Pirate XVI. 
Development of the Polliniwn. 
Fig. 1. Very early stage of one half of an anther, seen in transverse section, showing division of the 
single archesporial cell into inner and outer segments, the former, which alone is shaded, con- 
stituting the primary mother cell of the pollen, ртс; z, the outer segment ; epi, the epidermis 
covering the anther; par, ground-tissue of the anther. The fibro-vascular bundle of the 
connective is not yet visible. 
2. A slightly later stage, also seen in transverse section. The primary mother cell, pme (alone 
shaded), has now longitudinally divided into two mother cells. The outer segment, z, of the 
previous stage, has divided tangentially into three rows of cells, and these, again, vertically. - 
The innermost row of these adjacent to the primary mother cell is the tapetum proper, tap. 
З. A later stage. The two mother cells of the previous stage have now divided each in a longi- 
tudinal plane, so that four mother cells, m с, are seen ; ѓар, tapetum proper ; Тар“, segments 
cut off from the surrounding parenchyma, by which the tapetum is completed on the external 
side. The hypodermal layer of the previous stage, formed from z, has now become divided into 
two by longitudinal division. 
4. Later still. The mother cells, mc, have become more numerous by longitudinal division of each of 
the four seen in the previous stage into two; and they have at the same time become longer. 
At this stage the contents of the loculus very closely resemble those of Zostera immediately before 
the formation of the pollen-grains. The other parts as before. The fibro-vascular bundle of 
_ the connective, v, has become visible a little before this stage. 
5. Longitudinal section of an anther-lobe when the stage represented in the last figure (fig. 4) 
: has been reached. The parts are lettered as in the previous чок 
ce SECOND SERIES. —BOTANY, УО HH _ o 
