Oct. i Sl 1920 Influence 0} Cold in Stimulating Growth of Plants 157 



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. Con- 

 sider 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 attract- 

 ing sugar-loving insects which unconsciously carry pollen 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 observed to exude drops of sugar solution from certain 

 glands on the margins of the leaf and on the back of the midrib (PI. 28). 

 It is physically impossible for the sugar to have left the cells by osmosis. 

 The sugar serves no useful purpose to the plant through the attraction 

 of insects. The exudate certainly can not represent the elimination of a 

 waste product, for sugar is one of the substances most used by plants in 

 forming new tissues. I can conceive of no reason why the plant should 

 exude sugar except to relieve a dangerous physiological condition — 

 namely, the development of excessive osmotic pressures which would 

 burst the cells of the plant or in some other way derange its physiological 

 activities. I look upon such sugar glands as safety valves for the relief 

 of excessive osmotic pressures that are dangerous to the internal economy 

 of the plant. And not only is this conception applicable to extra-floral 

 nectaries in general, but it may serve also, in the case of floral nectaries, 

 to explain their origin. Having once arisen as osmotic safety valves, 

 the usefulness of the floral nectaries as an aid to cross-pollination would 

 then tend strongly to bring about their natural selection and perpetuation. 



7. The establishment of a dormant condition before the ad- 

 vent OF FREEZING WEATHER AND THE CONTINUATION OF THIS DOR- 

 MANCY THROUGH WARM PERIODS IN LATE FALL AND EARLY WINTER ARE 

 PROTECTIVE ADAPTATIONS OF VITAL NECESSITY TO THE NATIVE TREES 

 AND SHRUBS. 



A little consideration will show how important the principle of chill- 

 ing is to those species of trees and shrubs which are subjected each 

 year to several months of freezing weather. If they were so consti- 

 tuted as to start into growth as easily in the warm days of late fall as 

 they do in the warm days of early spring many species would come into 

 flower and leaf in those warm autumn spells that we call Indian summer, 

 and the stored food that the plant required for its normal vigorous growth 

 in the following spring would be wasted in a burst of new autumn growth, 

 which would be killed by the first heavy freezes and would be followed 

 by a winter of weakness and probable death. But when two or three 



