132 



HISTORY OF THE VEGETABLE KINGDOM. 



seems to be a sudden decay of the vital principle, 

 like that produced in animals by lightning. In 

 an hour's time, from some unknown causea joint, 

 a whole branch, or sometimes an entire plant of 

 the nopal, changes from apparent health to a 

 state of putrefaction or dissolution. One minute 

 its surface is verdant and shining, the next it 

 turns yellow, and aU its brilliancy is gone. On 

 cutting into its substance, the inside is found to 

 have lost all cohesion, being quite rotten. The 

 only remedy in this case, is speedy amputation 

 below the diseased part. Sometimes the force 

 of the vital energy makes a stand as it were, 

 against the encroaching disease, and throws off 

 the infected joint or branch, just as we find the 

 vital powers in animals overcoming the effects 

 of mortification. 



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 dai'k rooms, 

 or between great masses of stone, or in the clefts 

 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 

 colour. Etiolation may also occur from the 

 depredation of insects nestling in the radicle, 

 and consuming the food of the plant; and thus 

 debilitating the vessels of the leafj so as to render 

 them unsusceptible to the action of light. This 

 is said to be often the case with the radicles of 

 sacale cereals; and the same circumstances may 

 also arise from poverty of soil. 



Suffocation. It occasionally happens that ex- 

 traneous substances may so obstruct the pores 

 of the epidermis, as to prevent the free exhala- 

 tion of the juices, and thus produce the disease 

 called suffocation. Sometimes it is caused by 

 the accumulated growth of the lichens on the 

 bark extending over the whole plant, as is seen 

 frequently in fruit trees. If the young and suc- 

 culent branches are thus coated, the proper func- 

 tions of the bark are interrupted and decay, and 

 death of the tree ultimately takes places. Fruit 

 trees, on this account, should be carefully cleared 

 of these parasites. 



A similar effect is also produced by insects 

 which feed on the sap or shoot. Thus the aphis 

 or plant louse, accumulates in such myriads on 

 tender shoots, as to exclude the air altogether, 

 and consume the juices. The coccus hesperidum, 

 and acarus tellarius, are parasitical insects, which 

 infest hot-house plants, the latter by spinning 

 a fine and delicate web over the leaf, and thus 

 preventing the access of atmosphei-ic air. Some- 

 times the disease is occasioned by an exudation 

 of juices, wliieh thicken on the surface of the 



stalk, so as to fonn a crust investing it as a 

 sheath, and preventing its further expansion. 

 Dr Keith writes that, on the 7th July, 1816, he 

 observed some stalks of a grass partly enveloped 

 with a crust, not unlike a piece of dried orange 

 piU, particularly when viewed through the mi- 

 croscope. The part thus enveloped proved to 

 be that in which the spike was yet contained 

 within its sheathing leaves. The crust which 

 thus totally locked up and suffocated the spike, 

 extended from about one and a half to two inches 

 in length, surmounted by the terminating leaf, 

 whose base it also invested, thus giving to the 

 grass the appearance of a typha in miniatm-e. 

 On examining this crust more minutely, it 

 seemed to consist of thousands of yellowish 

 globules, imbedded in a sort of ground resem- 

 bling mortar. But in some species the cmst was 

 much paler, and not unlike the boletus medulla- 

 panis in a recent state. It not only invested the 

 outer leaf, but also the inner, though sheathed 

 by the outer; and the spike, though sheathed by 

 the inner leaf. The ear was so totally consumed, 

 or so imperfectly formed, that the species of 

 grass could not be ascertained till afterwards 

 that a sound ear found showed it to be holcus 

 lanatus. If this cmst is not originally caused 

 by the puncture of insects, it is at least selected 

 as a fit nidus for depositing their eggs. For in 

 looking at some specimens about a week after, 

 several were found in which the surface of the 

 crust was disfigured, with a, sort of protuberant 

 blister, which, when opened up, contained a 

 maggot. And even in unsheathing, an ear which 

 was thus locked up, and apparently inaccessible 

 to insects, a small black fly occupied the interior. 

 Sometimes the disease is occasioned from want 

 of an adequate supply of nourishment, as derived 

 from the soil in which the lower part of the 

 plant is the best supplied, while the upper part 

 is starved. Hence the top shoots decrease in 

 size every succeeding year, because a sufficient 

 supply of sap cannot be obtained to give them 

 their proper development. This resembles what 

 takes place in animal life, when the action ot 

 the heat becomes too feeble to propel the blood 

 through the whole of the system, for then the 

 extremities are the first to suffer. It may per- 

 haps also account for the fact, that in bad soils, 

 and unfavourable seasons, when the ear of barley 

 is not wholly perfected; yet a few of the lower 

 grains are always completely developed, which 

 not only shows the superintending care of Provi- 

 dence for the preservation of the species, but 

 points out also the efiicient cause. 



Contortion. The leaves of plants are often 

 injured from the puncture of insects, so as to 

 induce a sort of disease that discovers itself in 

 the folding up or contortion of the margins, or 

 ■\\Tinkled appearance of the surface. The leaves 

 of the apricot, peach, and nectarinc,are extremely 



