Guar. IL THE PROCESS OF AGGREGATION. 61 
under a high power, is the appearance of the finest 
granules in the fiuid within the cells, making it 
slightly cloudy. These granules soon aggregate into 
small globular masses. I have seen a cloud of this 
kind appear in 10s. after a drop of a solution of car- 
bonate of ammonia had been given toa gland. With 
dark red leaves the first visible change often is the 
conversion of the outer layer of the fluid within the 
cells into bag-like masses. The aggregated masses, 
however they may have been developed, incessantly 
change their forms and positions. They are not filled 
with fluid, but are solid to their centres. Ultimately 
the colourless granules in the protoplasm which flows 
round the walls coalesce with the central spheres or 
masses; but there is still a current of limpid fluid 
flowing within the cells. As soon as the tentacles 
fully re-expand, the aggregated masses are redis- 
solved, and the cells become filled with homogeneous 
purple fluid, as they were at first. The process of re- 
dissolution commences at the bases of the tentacles, 
thence proceeding upwards to the glands; and, there- 
fore, in a reversed direction to that of aggregation. 
Aggregation is excited by the most diversified 
causes,—by the glands being several times touched,— 
by the pressure of particles of any kind, and as these 
are supported by the dense secretion, they can hardly 
press on the glands with the weight of a millionth of 
a grain,*—by the tentacles being cut off close beneath 
* According to Hofmeister (as 
quoted by Sachs, ‘ Traité de Bot! 
1874, p. 958), very slight pres- 
sure on the cell-membrane arrests 
fhamediately the movements of 
the protoplasm, and even deter 
mines its separation from the 
walls. But the process of aggre- 
gation is a different phenomenon. 
as it relates to the contents of the 
cells, and only secondarily to the 
layer of protoplasm which flows 
along the walls; though no doubt 
the effects of pressure or of a 
touch on the outside must be 
transmitted through this layer. 
