THE STOMATA AND TRANSPIRATION OF OAKS

Abstract

Introductory Stomata and transpiration are of fundamental importance in the water relations of plants. It is evident that the loss of moisture in transpiration is equally as important as absorption of water by the root system in plant survival. Our mesophytic plants must have stomatal protection against excessive loss of moisture during drought. This is of paramount importance to us in our conservation work, where trees are being used as windbreaks, and to conserve moisture. Most of our knowledge regarding stomata has to do with herbaceous plants, as indicated by texts in plant physiology (MILLER, 7, and others). Growth habits and therefore soil conditions are not the same for leaf formation on herbaceous plants as on trees. On trees most of the leaves grow simultaneously in the spring while on herbaceous plants they appear over a longer period. In herbaceous plants stomata are commonly on both leaf surfaces, but tree leaves have them usually only on the lower surface. Nearly all the water loss of plants occurs through the stomata. Some is lost through the epidermis, and since the upper epidermis has tlhicker cell walls, it is probable that a little more water loss occurs through the lower than through the upper epidermis. Water passes freely through the sto-mata when open; in fact BROWN and ESCOMBE (2) have shown that water may be lost through the stomata as rapidly as though it were evaporating from a free water surface instead of through many small pores. SAYRE (12) has shown that the rate of transpiration depends more on the perimeter of the opening, than on its area, which is very important in the study of partly closed stomata. It is evident from their shape that the beginning of closure changes the perimeter very little, and only the last few per cent. of closure will in many cases affect the transpiration rate, as found by JEFFREYS (4). Methods The relation of the stomata to transpiration was measured by the cobalt chloride paper method, since it seemed to offer a more accurate measure than the actual observational measurement of the final stag,es of closure in percentages. Mature leaves on one to three year old Q. palitstris, Q. rubhra, and Q. prinus seedlings grown outdoors, some in pots and some in seed beds, were used. Readings were taken at intervals of two hours or less over twenty-four hour periods. 795