PROTOPLASM THROUGH THE WALLS OF VEGETABLE CELLS. 
831 
zygomeris, and Phaseolus multifiorus. I do not intend in the present paper to enter 
into any discussion with regard either to the nature of irritability or the phenomena 
of movement of which these plants serve as illustrations, but merely to confine myself 
to such structural detail as is necessary for the clear comprehension arid significance 
of my results. The principal literature of the subject has been collated by Pfeffep,.,* * * § 
to whose researches and those of Sachs! we owe the greater part of our knowledge of 
what is one of the most interesting phenomena of plant life.J 
Mimosa pudica .—As a rule the main pulvini at the base of the petiole of the leaf 
were chiefly made use of on account of their larger size and consequent easier 
manipulation. The secondary and tertiary pulvini, however, gave the same result. 
Thin longitudinal and, as far as possible, axial sections of the fresh material 
were taken, since the unequal contraction and puckering-up of the tissue due to 
the tensions produced by the violent action was not so great as in transverse 
sections. 
As regards its anatomical structure the pulvinus shows a thin vascular bundle 
surrounded by a thick layer of parenchymatous cells. The epidermis is not pro¬ 
nounced and the epidermal cells have undergone very little, if any cuticularisation. 
For the most part confined to the underside of the pulvinus are several long stiff 
multicellular hairs. 
Immediately under the epidermis the cells are small, as are those immediately 
surrounding the bundle, and between these two layers occur the cells of maximum 
size (see Plate 68, figs. 1 and 3). From the hypodermal cells inwards the intercellular 
spaces, which are at first inconspicuous, become more and more apparent, until in these 
cells around the bundle itself a system of large communicating air spaces exist (Plate 68, 
figs. 3 and 4). The vascular bundle is arranged on the concentric type, the phloem 
being outermost and surrounding the xylem. In the phloem the walls of the prosen- 
chymatous cells are greatly thickened and very highly refractive ; the middle lamellae 
between them are also almost inconspicuous (see Plate 68, fig. l), the structure of 
which is similar to that of Mimosa , The cell-walls of the upper half of the pulvinus 
are thicker than those of the lower, which moreover is the side towards which the 
bending takes place, and this rule is followed in the secondary and tertiary pulvini 
also, viz. ; that the cells of the side which becomes concave on bending, have always 
thinner walls than the side which becomes convex, so that whereas in the main 
pulvini the underside has the thinner walls, in the pulvini of the leaflets the reverse 
is the case. The parenchymatous cells each contain a number of chlorophyll granules 
and a nucleus, One or more drops of tannin are also present,§ which can be well seen, 
* Pfeffer, ■ Physiologiste Untersuchungen,’ 1873, i. id. ; ‘ Die periodischen Bewegungen der 
Blattorgane,’ 1875 ; see also ‘ Pflanzen Physiologie,’ 1880. 
t Sachs’, ‘ Handb. der Exp. Pbys.,’ 1866, p. 479, et seq. 
+ See also Darwin, ‘Movements of Plants,’ 1880. 
§ I could not detect the special pellicle mentioned by Pffefer. See Sachs’ Text-book, p. 889. 
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