2 $6 THE OLIVE LEAF. CHAP. 



to accomplish this feat, and therefore perishes in the 

 attempt. 



Not the least extraordinary fact in the strange history 

 of these products is the great variety of effects produced 

 by apparently the same instruments and agencies. Out 

 of the juices of the same oak are formed galls of very 

 dissimilar appearance by the punctures of the different 

 gall-flies. The cause of this diversity is still one of the 

 unexplained mysteries of nature. Not only are the 

 forms different, but the tissues are also different. In 

 some the structure is soft and juicy; in others hard and 

 woody outside, having within a layer of cellular tissue 

 filled with starch grains, which afford food to the larvae. 



Galls are morbid growths, caused, as we have seen, 

 by the puncture of a minute fly. The abnormal vital 

 action that is set up by the irritation of the wound 

 produces a change of organization, from which results a 

 complete change in the external form and even in the 

 internal substance of the part attacked. The cellular 

 tissue swells, the parts which were naturally long be- 

 come round, and starch, which is formed in those parts 

 of a plant whose vital activity has been suspended, is 

 deposited. There is a curious parallelism between the 

 gall-nut and the acorn, the animal and the vegetable 

 product. The gall-nut goes through the same changes 

 with the acorn, remaining green as a simulated fruit 

 through the summer, but assuming in autumn the russet 

 hue of the foliage and fruit. In winter the acorn falls 

 from the tree and is buried in the soil that it may produce 

 the young sapling ; in winter the gall-nut remains upon 



