MR. T. H. CORRY ON ASCLEPIAS CORNUTI. 199 
‚ to extend through this margin for the greater part of its length. The chink which it 
produces is a regular cut, with irregular lacerated margins. Through this chink a great 
number of extremely slender, more or less elongated, filaments issue іп a forward дігес- 
tion. When these filaments are examined they are seen to be transparent, membranous, 
cylindrical tubes, consisting of a cellulose wall containing granular protoplasm together 
with numerous vacuoles. Between these tubes a greater or less abundant amount of 
granular matter may sometimes be detected. Тһе tubes lie side by side, forming а sort of 
skein, consisting of one, or sometimes of two limbs. They are easily observable, even with 
the naked eye. By dilating the aperture through which these tubes come forth, which 
may readily be done, and so exposing the interior of the pollinium, each of these tubes is 
seen to be a prolongation froma grain of pollen. The grains which have produced pollen- 
tubes are seen to possess very nearly the original form which they had when the pollinium 
was fully mature, save that they are swollen and slightly more transparent, the latter 
fact being due to their having lost a certain amount of the granules and vacuoles with 
which their protoplasm was previously crowded. It is also discovered that the production 
of the pollen-tubes has taken place from different sides of the pollen-grains forming the 
three rows previously described * as existing in the pollinium. From the grains of the 
row which abuts most closely on the more curved edge the pollen-tubes are produced on 
that side which lies away from the convex edge, except in the case of those grains which 
lie opposite the chink. The lacerated margins of the chink are themselves produced by 
the bursting of the transformed wall of the special pollen mother-cells surrounding the 
pollen-grains on this aspect, as well as of the general outer coat of the pollinium. In the 
case of the grains opposite the ehink, and, indeed, of all the other grains forming the 
pollinium, the pollen-tubes are definitely produced from the side which lies facing the 
chink, i. e. the internal side, through Ше сит membranes surrounding them which they 
rupture. The grains forming the central layer, when they produce their tubes, generally 
separate at the same time from one another. Each is, however, still surrounded by the 
wall of the special mother-cell, which at a considerably earlier period became chemically 
altered into cutin, and assumed a pale yellow colour, and which has only ruptured in this 
case along the part of each where the pollen-tube is produced. This cutin covering may 
be easily removed from the grain which it incloses without any further change taking 
place. The cutin walls which bound the internal faces of the two superficial rows of 
cells containing the pollen-grains rupture on the production of tubes from their contained 
grains; and as the separation from them of the special mother-cells of the central row 
with their contents (viz. pollen-grains) and cutin walls takes place at the same time, this 
rupture occurs to such an extent that the remains of the cutin walls which bound these 
superficial cells internally cannot be easily distinguished. A very few pollen-grains may 
perhaps be detected in the pollinium which have not produced pollen-tubes. Some of the 
pollen-grains may also be found discharged from the pollinium through the chink still 
unchanged, while some of these, again, have begun to produce pollen-tubes like those 
which are still inclosed. The pollen-tubes are directed from all parts of the pollinium 
* «(n the mode of development of the pollinium in Asclepias Cornuti,” Linn. Trans. Bot. ser. ii. vol. 2, pp. 75- 84. 
202 
