82 MR. T. Н. CORRY ON THE DEVELOPMENT 
which the very presence of nuclei of any kind whatever had not been previously 
detected. 
I consider the smaller nucleus of the Asclepiad pollen-grain to be the representative 
of what Elfving terms the © vegetative nucleus," and others have designated as Ше 
* passive nucleus," which nucleus is genetically the last remnant of the male рго- 
thallium of a vascular Cryptogam type, such as Equisetum, while the larger nucleus, 
equivalent to the “ active nucleus," is genetically the last remnant of the antheridium 
of such a type. 
In shape the pollen-grains are always nearly spherical, though usually slightly 
angular, so as to be really irregularly polyhedral (Pl. XVI. fig. 11); their membrane is, 
as previously stated, single, very thin at first, ultimately becoming thicker, smooth, 
hyaline, and transparent, and formed of unchanged cellulose. "There is at this stage no 
appearance whatever of the tubes which are afterwards produced. 
Strasburger, in his most recently published work *, mentions the fact that he has 
observed the presence of only a single coat in the pollen-grains of the following plants 
—Gaura biennis, L., Clarkia elegans, Dougl., Senecio vulgaris, L., Cobea scandens, 
Cav., Allium, L., Naias major? t, and Orchids; and the same phenomenon was first 
described by Fritszche |, and has long been known to occur in Zostera, L., while 
Asclepias Cornuti, Decaisne, must now be added to this interesting list of exceptions to 
what is otherwise the universal rule among phanerogamous plants. 
The ultimate changes and fate which the tapetal membrane undergoes appear to be as 
follows :—The cells composing it which lie on the outer side of the anther divide each by 
means of a vertical tangential wall, parallel to the original tangential walls of the cell, so 
that the membrane becomes two cells broad on this side. Those tangential walls which 
are furthest from the pollinium in that row of limiting cells which is next the cavity 
of the loculus, together with the adjacent portions of the radial walls of these 
cells, become broken down and disintegrated. The tangential walls, on the other 
hand, which are nearest the pollinium, together with the internal portions of 
the said radial walls, persist for some time, forming a continuous membrane, sur- 
rounded by a layer of small cells. These latter, on the outer side of the anther, аге 
segments from those cells which completed the tapetum proper on this aspect, and were 
themselves derived from the parenchyma, while on the inner side of the anther they 
constitute simply that row of cells which were formed immediately external to the 
tapetum proper, at the same time that it was differentiated, and which have persisted. 
Such is the condition immediately prior to the opening of the two anther-loculi to 
expose the pollinia. The mode in which the dehiscence of the anther takes place will 
be fully described in a subsequent paper, the result being that the whole of the 
* Ueber den Bau und das Wachsthum der Zellhiute: Jena, 1832. 
t Hofmeister (Neue Beitrüge, 1859, part ii.) describes the existence of **a very thin but distinet extine ” jn the 
pollen-grains of this species ; but in his figure (pl. i. fig. 11) he represents this extine as extending with the intine 
along the pollen-tube produced from the grain! It is therefore highly probable that Strasburger's observation is 
more accurate. ~ 
+ * Ueber den Pollen,” Mémoire présenté à l'Académie Impériale de St. Pétersbourg, iii. 1837. 
