214 



(7) The more or less complete destruction of flowers, leaves, 

 twigs, and young stems, by hail. The damage done 

 depends largely on the state of development of the 

 plant. While mature leaves may only be penetrated 

 by the hail-stones, giving rise to so-called " shot- 

 holes," the injury being more or less local, tender 

 young leaves may be torn into shreds and the tree 

 completely defoliated. Such complete defoliation 

 results in the loss of valuable food materials con- 

 tained in the young leaves, while the tissues have 

 to be depleted of their food reserves for the for- 

 mation of fresh foliage, this reduction in the avail- 

 able food supply resulting in loss of increment and 

 diminished production of flowers and seed. 



of 190. All plants may be killed by 



sufficiently strong light, and the fact that disease-produc- 

 ing bacteria may be killed by a sufficiently long exposure 

 to sunlight is obviously of great practical importance. In 

 green plants the chlorophyll corpuscles are usually more 

 subject to injury than the rest of the protoplasm. If the light 

 is too intense these corpuscles lose their power of assimila- 

 tion and may become permanently bleached. The necessity 

 of protecting the chlorophyll corpuscles from intense light is 

 indicated by the presence of colouring matters such as antho- 

 cyanin, to which the red or purple colour of many young 

 leaves is due, and which acts as a protective screen to the 

 chlorophyll, by the general absence of chlorophyll in the 

 epidermis,[and by the fact that the chlorophyll corpuscles when 

 exposed to strong light arrange themselves in such a way in the 

 cells as to be least exposed to its action. 



On the other hand, deficiency of light may be no less injurious. 

 In the absence of light, green chlorophyll is as a rule not 

 formed and, the manufacture of food materials in the leaves 

 being impossible, the death of the plant by starvation ensues. 

 We know, for instance, how easily so-called ' ' light-demanding ' : 

 species may be killed by shade. Insufficient light causes the 

 diseased condition known as etiolation, which is characterised 

 by the development of abnormally long and thin internodes and 

 small, or very thin, yellowish leaves, with unusually watery 

 tissues. The thickness and rigidity of the cell walls is also 

 diminished, and the laying of cereals is due to the shading of 

 the lower portion of the haulms which thus become too weak 

 to support the weight of the plants. Unless the etiolated 



