298 DISEASES OF TREES 



produced under such circumstances, are peculiar in possessing 

 an epidermis which is comparatively non-tuberous.* On this 

 account it is ill-adapted to prevent the excessive transpiration 

 which is encouraged by air-currents and a dry atmosphere, and 

 such plants wither or lose a portion of their leaves prematurely. 



A sudden accession of light in too large quantity has also a 

 prejudicial influence on the health of plants, and especially 

 on the leaves of trees, whether dicotyledons or conifers. 

 Under normal conditions the chlorophyll-corpuscles protect 

 themselves against the action of too bright light, which would 

 destroy their green colouring matter, by so arranging themselves 

 in the cells of the leaf that their narrow edge only is exposed to 

 the intense illumination. The leaves of plants that have been 

 reared in shade become yellow or brown when suddenly exposed 

 to the action of direct sunlight. In such a case, however, it is 

 always difficult to determine how much of the damage is to be 

 ascribed to the accelerated transpiration induced by the intense 

 sunlight, and to the consequent withering of the cells. 



On the other hand, it is a familiar fact that pathological 

 phenomena may also be induced by deficiency of light. A plant 

 that has grown up in unrestricted light possesses a certain stock 

 of the products of metabolism which have not, so far, been 

 utilized in the construction of cells. These may take the form 

 of reserve materials which have been stored up in the plant, or 

 of active plastic substances which are distributed throughout the 

 leaves and organs of the stem. By means of these substances a 

 plant is able to grow for a certain time even without light, until, 

 in fact, the substances have been utilized and the supply has 

 been exhausted. Shoots and leaves that have been produced 

 in the dark are, however, abnormally constructed. They are 

 spindly, and " drawn," and display the phenomenon of so-called 

 etiolation. As chlorophyll can normally be produced only under 

 the action of light, and as the supply of nutritive substances is 

 insufficient, the shoots and leaves are yellowish and not properly 

 developed. Seeing that light cannot exert its retarding influence, 

 the shoots become abnormally elongated. Such drawn shoots, 



* [There are other anatomical differences also in the cellular tissues of 

 such shade leaves, as Stahl has shown, which are calculated to make them 

 less resistent. ED. 



