DISEASES OF VEGETABLES. 



133 



liable to be thus affected in the months of June 

 and July. Tlie leaf that has been punctured 

 soon begins to assume a rough and wrinkled 

 figure, and a reddish and scrofulous appearance, 

 particularly on the upper surface. The margins 

 roll inwards on the under side, and enclose the 

 eggs which are scattered irregularly on the sur- 

 face, giving it a blackish and granular appear- 

 ance, but without materially injuring its health. 

 In the vine, the substance deposited on the leaf 

 is whitish, imparting to the under surface a 

 frosted appearance; but not occasioning the red 

 and scrofulous aspect of the upper surface of 

 the leaf of the nectarine. In the poplar, the 

 eggs, when first deposited, resemble a number of 

 small and hoary vesicles, containing a sort of 

 clear and colourless fluid; the leaf then becomes 

 reflected and folded up, enclosing the eggs, and 

 exhibits a few reddish protuberances on the upper 

 surface. The embryo is nourished by the con- 

 tained fluid, and the hoariness is converted into 

 a fine cottony down, which for some time envel- 

 opes the young fly. The leaf of the lime tree 

 in particular is liable to attacks from insects 

 when fully expanded, and hence the gnawed ap- 

 pearance it so often exhibits. The injury seems 

 to he occasioned by some species of puceron, de- 

 positing its eggs in the parynchema, generally 

 about the angles that branch ofl^ from the mid- 

 rib. A sort of down is produced, at first green, 

 and afterwards hoary ; sometimes in patches, 

 and sometimes pervading the whole leaf, as in 

 the case of the vine. Under this covering the 

 eggs are hatched, and then the younginsect gnaws 

 and injures the leaf, leaving a hole or scar of a 

 burnt or singed appearance. Sometimes the 

 upper surface of the leaf is covered with clusters 

 of wart-like substances. These seem to have 

 their origin from a puncture made on the under 

 surface, on which a number of openings are 

 discoverable penetrating into the warts, which 

 are rotten and villous within. 



The punctures causing the gall-nuts are oc- 

 casioned by insects chiefly of the genus Ci/nips. 

 These punctures are made in some vigorous part 

 of the plant, as the leaves, leaf stalks, young 

 stem or branches; or more rarely in the calyx 

 or germen. The parent insect deposits its egg 

 there, which is soon hatched, and in consequence 

 of the perpetual irritation occasioned by the 

 young maggot feeding on the juices of the plant, 

 the part where it is lodged acquires a morbid 

 degree of luxuriance, frequently swelling to an 

 immoderate size, and assuming the most extra- 

 ordinary and whimsical shapes. This often 

 happens to the shrubby species of hawkweed 

 and wnbellatum, whose stems in consequence 

 swell into oval knots. Several different kinds 

 of galls are borne by the oak, as those light 

 spongy bodies as big as walnuts, commonly 

 called oak apples : a red bsrry-like excrescence 



on its leaves, and the very astringent galls brought 

 from the Levant for the purposes of dyeing and 

 making ink, which last are produced by a spe- 

 cies of oak different from the British. The 

 common dog rose frequently bears large moss- 

 like balls, in whose internal parts numerous 

 maggots are always to be found, till they become 

 the winged a/nips rosas, and cat their way out. 

 Many willows bear round excrescences as large 

 as peas, on their leaves. The mastic tree is often 

 laden in the south of Europe with large red 

 hollow finger-like bodies, swarming internally 

 with small insects. The young shoots of salvia 

 pomifera,a.n& other species, in consequence of the 

 attacks probably of some cynips, swell into 

 large juicy balls very like apples, and even 

 crowned with rudiments of leaves resembling 

 the calyx of that fruit. These are esteemed in 

 the Levant for their aromatic and acid flavour, 

 especially when prepared with sugar. It may 

 be remarked that all these excrescences are more 

 acid than the plant which bears them, and also 

 generally inclined to turn red; the acid is partly 

 the acetous. 



Consumption. This consists in a gradual de- 

 crease of the energy of the vegetable functions, 

 till at last decay and death occurs, and may arise 

 from a barren or unfavourable soil, from climate, 

 careless planting, or too frequent blossoming 

 exhausting the strength of the plant. Excess 

 in drought, or dust lodging on the leaves, or the 

 fumes of deleterious matters floating in the at- 

 mosphere of manufactories, occasions the same 

 decay, and not unfrequently the attacks of nu- 

 merous minute insects. 



There is a malady which frequently attacks 

 the pine tree, called teredo pinorum, which seizes 

 on the alburnum and inner bark chiefly, and 

 seems to proceed from long-continued drought, 

 or from frost suddenly succeeding mild or warm 

 weather, or heavy winds. The leaves assume a 

 tinge of yellow bordering on red, a great num- 

 ber of small drops of resin exude from the mid- 

 dle of the boughs, of a putrid odour, the bark 

 peels off, and the alburnum presents a livid aj)- 

 pearance. The tree swarms with insects, and 

 the disease is incurable, inducing inevitably the 

 total decay and death of the individual. 



Natural decay. We have thus enumerated the 

 principal diseases to which plants are liable, whe- 

 ther from external injuries, or internal derange- 

 ment of structure in functions. Yet though a 

 ! plant should escape all these, still a period will 

 t arrive when its several organs vvill begin to ex- 

 perience the approaches of a natural decay, the 

 vital energies will at last cease to act, and the 

 plant will moulder into its component elements. 

 We thus find that in the vegetable, as well as 

 the animal kingdom, there is a limit or term set 

 beyond which the individual cannot pass, al- 

 though the period of existence varies as much 



