138 DISKASK IX I'LAXTS. 



Some interesting facts bearing on the matters 

 here under discussion, have been obtained from 

 the study of Galls, the curious outgrowths found 

 on many plants and due to the action of insects. 



A typical gall exhibits three distinct and 

 characteristic layers of tissue surrounding the 

 hollow chamber in which the larva of the insect 

 lies, viz., an outer layer of soft cells forming a 

 parenchyma covered with an epidermis, and fre- 

 quently also with a layer of cork ; an inner 

 stratum consisting of very thin-walled delicate 

 cells filled with protoplasmic and reserve food- 

 materials on which the larva feeds ; and between 

 the two a more or less definite layer of thick- 

 walled sclerenchyma cells which serve as a pro- 

 tection against accidents to the larva as the outer 

 layer shrivels or rots, or if it is exposed to the 

 attack of marauders. This layer may be absent 

 from galls which have a short life only. Vascular 

 bundles run into the outer layer from the leaf- 

 veins or the stele of the shoot, etc. Such galls 

 abound in tannin, and are frequently of use in 

 the arts on this account : they also contain starch, 

 and proteid substances and crystals of calcium 

 oxalate. When the larva has consumed the 

 stores of food material and reached the adult 

 stage it eats its way out and escapes. 



The growth of such a gall is preceded by the 

 laying of an &^^ on or in the embryonic tissue 

 of a leaf, stem, or other young part, and it 

 is interesting to note that only organs in the 

 meristematic stage can form galls, and that it is 



