600 
PRESIDENTS ADDRESS. 
layers of cells on the under part of the pulvinus, the cushion-like 
expansion of the base of the petiole of leaves. These cells almost 
immediately lose their water, and the leaf falls down. In the case 
of the leaflets, it is the cells on the upper part of the pulvinus that 
are affected, and the leaflets approximate each other. 
During the growth and development of plants, a great many 
physiological actions play their part, and we have to consider what 
the determining forces are that enable plants to assume certain 
forms, and how it is that some plants, on account of their inability 
to maintain an upright position, and lead an independent life, have 
to seek the help of others or some other support. Every plant in 
its start in life has an innate feeling of its own proportion and 
character, and is able to call into play, as it requires, those 
regulatory forces that will give the required result. The physio- 
logical outcome of all these forces is due to the vital property of 
irritability possessed by the protoplasm. The living cells of a plant 
are all united into one sympathetic whole by the connecting threads 
of protoplasm, which penetrate the cell walls, and which were 
first demonstrated by Gardiner. By reason of this arrangement, 
sympathetic responses to any stimulus are noted and conveyed to 
any part of the organism. 
The various stimuli that act upon an organism are, light, 
heat, chemical actions, and gravity, and in addition, wounds, 
some salts, as potash, and many others. Gravity exercises 
the greatest force in the development of a plant, and, in 
order to counteract this force, climbing plants have to employ 
certain expedients in the form of sensitive organs, that are ready 
to respond to an external stimulus. Examples of this are seen 
in the petioles of Clematis, and the tendrils of leguminous 
plants. It is only one half of the tendril that is sensitive, and 
this enables it to embrace tightly the support. The tendril of 
Ampelopsis is only sensitive at the end, and from the sustained 
stimulus of pressure, a sucker is developed which gives a firm 
support. We may complain of the prickles of brambles, but it is 
only by the help of these, responding to the stimulus of touch, that 
the stem is able to attach itself to supports in its struggle through 
