VEGETABLE PHYSIOLOGY. 79 



Mexico is also subject to a sort of gangrene that begins with a 

 black spot, and extends till the whole leaf or branch rots off or 

 the plant dies. 



But plants are sometimes affected with a gangrene by which 

 a part becomes first soft and moist, and then dissolves into foul 

 ichor. This is confined chiefly to the leaves, flowers and fruit. 

 Sometimes it attacks the roots also, but rarely the stem. It 

 seems to be owing in many cases to too wet or too rich a 

 soil ; but it may originate in contusion, and may be caught by 

 infection. 



But the nopal is subject also to a disease called by Thiery 

 la dissolution, and considered by Sir J. E. Smith as distinct 

 from gangrene. A joint of the nopal, or a whole branch and 

 sometimes an entire plant, changes in the space of a single hour 

 from a state of apparent health to a state of putrefaction or 

 dissolution. Now its surface is verdant and shining, and in an 

 instant it changes to a yellow, and its brilliancy is gone. If the 

 substance is cut into, the parts are found to have lost all cohesion, 

 and are quite rotten ; the only remedy is speedy amputation 

 below the diseased part. Sometimes the vital principle collecting 

 and exerting all its energies, makes a stand as it were against 

 the encroaching disease, and throws off the infected part. 



8. Etiolation. Plants are sometimes affected by a disease 

 which entirely destroys their verdure, and renders them pale and 

 sickly. This is called etiolation, and may arise merely from 

 want of the agency of light, by which the extrication of oxygen 

 is effected, and the leaf rendered green. And hence it is that 

 plants placed in dark rooms, or between great masses of stone, 

 or in the cleft's of rocks, or under the shade of other trees, look 

 always- peculiarly pale. But if they are removed from such 

 situations and exposed to the action of light, they will again 

 recover their green color. Etiolation may also ensue from the 

 depredation of insects, nestling in the radicle, and consuming the 

 food of the plant, and thus debilitating the vessels of the leaf so 

 as to render them insusceptible to the action of light. This is 

 said to be often the case with the radicles of Secale cereale or com- 

 mon rye, and the same result may also arise from poverty of soil. 



