PROTOPLASM THROUGH THE WALLS OP VEGETABLE CELLS. 
835 
plasm of the cell. Each pair of processes with their above-mentioned broad ends and 
the cloud between them forcibly suggest a sieve-tube arrangement, and from analogy, 
as I shall point out later on, I believe that such is the case. I have, however, with 
the highest powers at my disposal been unable to resolve the stained structure 
traversing the pit membrane into fine lines as I had hoped to do, although the whole 
appearance is most strongly suggestive of a striation, the direction of which is parallel 
to the long axis of the pit. However, it seems certain that there is a protoplasmic 
communication which can be plainly seen, and is not complicated by the presence of 
a stained middle lamella. 
Thus it appears that from the epidermal cells right up to the last living bast fibre 
which impinges on the first dead vessel a direct continuity from cell to cell has been 
established, and that such a pulvinus may be regarded as a connected whole. 
Robinia pseudacacia .—As in Mimosa, thin axial, longitudinal sections of the main 
pulvini were examined. Fresh material was used in every case, and after treatment 
with sulphuric acid the sections were stained with either Hofmann’s violet and 
glycerine, or with Hofmann’s blue. In fundamental structure the pulvinus of this 
plant resembles that of Mimosa. Hough examinations show that it is much larger, 
and that its surface is quite smooth and free from hairs. The cells do not appear to 
be so freely pitted, nor is tannin so abundant. In many of the cells which are scattered 
about the tissue, and are smaller than their neighbours, are crystals of calcium oxalate, 
which can be well seen embedded in the protoplasm of the containing cell. The cells 
in certain cases possess more than one nucleus. The nuclei are large and well 
developed, and are brought into prominent view in the case of tissue which has been 
previously treated with picric acid. 
After treatment with sulphuric acid it can be seen that, in a well prepared section, 
the cells present very much the same appearance as those of Mimosa (Plate 68, fig. 7). 
The continuity existing between the processes is not as pronounced as in the former case, 
and the appearance of threads going straight and uninterruptedly through the pits is not 
so frequent (Plate 68, fig. 7). On the contrary, there is more indication of the exist¬ 
ence of a sieve-plate-arrangement, which is very marked in those cases which admit of 
successful observation. Frequently at the point of junction of two processes there is 
a distinct and well defined swelling which stains perceptibly lighter than the very 
darkly stained threads, which it connects one with the other. It can clearly be 
distinguished from the pit membrane and the middle lamella, and can almost certainly 
be resolved into a striated appearance, although the observation cannot perhaps be 
regarded as perfectly conclusive or perfectly satisfactory. In consequence of the 
presence of fewer pits on the cell walls the interconnecting protoplasmic processes 
are fewer in number than in Mimosa. The bast fibres present the same appearance 
as those of Mimosa, although the appearance, on the whole, is not so marked. 
The secondary pulvini display essentiallyt he same structure as the main pulvinus 
of the whole leaf, with the exception that the number of tannin cells is very 
