COLD AND GROWTH OF PLANTS — COVILLE. 287 
to expect that some of them will be found to accomplish this act in 
the same way as our long-dormant greenhouse plants, by the weak- 
ening of their cell membranes. This, I have endeavored to show, is 
in its effect substantially identical with chilling. 
6. The twigs of trees and shrubs after their winter chilling and the 
transformation of their starch into sugar may he regarded as mechan- 
isms for the development of high osinotic pressures which start the 
plant into growth. 
Food in the form of starch can not be utilized by a plant directly. 
The starch must be changed into sugar before it can be used in mak- 
ing new growth. But this transformation does more than make the 
starch available as food for the growing plant. It serves also to 
increase the tendency of the cells to swell and enlarge. In the form 
of starch the material is inert in the creation of osmotic pressures, 
but when transformed into sugar it becomes exceedingly active. Ac- 
cording to the rigid experimental tests of H. N. Morse and his asso- 
ciates, a normal solution of cane sugar at 32° F. has an osmotic power 
of 25 atmospheres of pressure. It has been demonstrated that there 
sometimes occur in the cells of plants osmotic pressures as high as 
30 atmospheres, or 450 pounds to the square inch, a pressure sufficient 
to blow the cylinder head off an ordinary steam engine. It can hardly 
be questioned that these or even much lower osmotic pressures take 
an important part in forcing open the buds of once dormant plants. 
We have evidence that there sometimes arise within the plant 
osmotic pressures of such intensity as to threaten the rupture of the 
cells. Consider the case of the exudation of drops of sugar solution 
from certain specialized glands. When this exudate of sugar occurs 
in flowers it is known as nectar and it serves a useful purpose to the 
plant by attracting sugar-loving insects which unconsciously carry 
I^ollen from flower to flower and accomplish the beneficial act of 
cross-pollination. But sugar solution is often exuded outside the 
flower, in positions, or at times, that preclude any relation to cross- 
pollination. For example, a blueberry plant during its spring 
growth, when a leaf has reached nearly full size, is sometimes ob- 
served to exude drops of sugar solution from certain glands on the 
margins of the leaf and on the back of the mid-rib. (See pi. 17.) 
It is physically impossible that the sugar has left the cells by osmosis. 
The sugar serves no useful purpose to the plant through the attrac- 
tion of insects. The exudate certainly can not represent the elimina- 
tion of a waste product, for sugar is one of the substances most used 
by plants in forming new tissues. I can conceive of Jio reason why 
the plant should exude sugar except to relieve a dangerous physio- 
logical condition, namely, the development of excessive osmotic 
pressures which would burst the cells of the plant or in some other 
