PLANT-CELLS 45 
(4) The capacity for growth and the production of off- 
spring (reproduction). These are especially characteristic 
of living protoplasm. 
(5) The possession of the power of originating move- 
ments not wholly and directly caused by any external 
impulse (automatic movements). Such, for instance, are 
the lashing movements of the cilia of minute plants and 
spores, Figs. 157, 169. 
(6) The power of shrinking or closing up (contractility). 
(7) Sensitiveness when touched or otherwise disturbed. 
This is shown by insect-catching leaves (Sect. 140), by the 
leaves of sensitive plants, and parts of certain flowers. 
The most remarkable and peculiar of the characteristics 
of protoplasm as above given are due to its possessing irri- 
tability. By this is meant the power to respond in some 
definite way to any suitable stimulus or exciting cause, 
acting from within or without the plant body. Some of 
the principal stimuli are gravity, heat, light, chemical sub- 
stances, and contact with solid objects. 
59. Osmosis. — In order to understand the selective 
absorption which constitutes a large part of the work of 
the roots of plants it is necessary first to study osmosis. 
This is the process by which two liquids separated by 
membranes pass through the latter and mingle. 
It is readily demonstrated by experiments with thin 
animal or vegetable membranes. 
EXPERIMENT X 
Osmosis as shown in an Egg. — Cement to the smaller end of an egg 
a bit of glass tubing about six inches long and about three-sixteenths 
of an inch inside diameter. Sealing-wax or a mixture of equal parts 
of beeswax and resin melted together will serve for a cement. 
