G60 EXTRACTS — HORTICULTURE. 
this case we are iiicliued to think, that the setds of the parasite remain about the 
buds and stems, until a fit period arrive for their evohition. — The red, termed 
" Ruhigo or rust" (Uredo salicis) occurs on the leaves and stems of many plants ; 
there is also a black kind, termed " Smut" (Uredo segetum.) It is most destruc- 
tive to wheat, oats, &c., which are also very often affected with the red kind. 
According to the experiments of Mr. Young on this subject, it appears, that the 
best and simplest method for the prevention of these parasitical diseases in corn, 
is to steep the seeds of affected plants in lime-water for twentj--four hours. 
When either the black or red parasite appears on other plants, treat them as for 
the Mildew. The leaves of several plants belonging to the kitchen-garden, viz. 
horse-raddish, cabbage, &c. are sometimes attacked by a parasitical plant (Urfedo 
Candida) in appearance not much unlike the "Mildew" to the naked eye, this 
disease has been found from experience to be highly infectious. The gooseberry 
tree is liable to be affected in spring by a curious parasitical disease, (CEcidium 
Grossularioe ; vide page 466.) The only remedy for which, as well as the former 
one, is the removal of the affected leaves, and this can be done generally with 
safety, as these diseases seldom involve many on the same plant at the same time. 
Discoloration of Leaves. Upon certain laws which regulate the propor- 
tion of Acid and Alkaline matter, depend the colours of the leaves of plants, 
and these we know to be as various as the shades in the rainbow, — a certain pro- 
portion of these matters appears to be allotted to each ; any deviation from 
which, generally more or less, injures the health of the plant. The most usual 
morbid change of colour in leaves is from green to white, either of the whole or 
only part of a leaf. Mr. Knight has observed, that plants with white leaves can- 
not survive long, and that variegated ones bear the deprivation of the light ill : 
but he believes there are many such which are neither in a state of disease nor 
debility. This indeed seems to be the case with some, but there are very few 
which will not be found to suffer in various degrees from the variegation. Mor- 
bid discoloration happens from various causes, which counteract the natural in- 
fluence of light on the leaves, aud it may ultimately be in consequence of the 
obliteration of the pores, or " culs de sac," according to Mr. Todd Thompson, 
by which respiration is performed. In such cases, the blanched portions may 
obtain a quantity of carbonic acid, from that absorbed by the green portion of 
the leaf; bxit when all white, the acid, it is possible, may be obtained by the 
roots or stems. However, it is agreed, that it is from the accunuilation of carbon- 
ic acid which takes place, such portions of the leaves being unable to decom- 
pose it, that the change of colour happens, and which, indeed, seems to varj' ac- 
cording to the excess of acid present. A want of proper nourishment, is a fre- 
quent cause of discoloration, also absence of light, and attacks of insects, as in 
the white and red tumours of the .4phis; again, from the distruction of roots or 
other vital parts ; and indeed, any thing which tends to injure the health of the 
plant generally, is apt to produce discoloration of the leaves. — 
Gangrene of Leaves. — All the diseases described are apt to cause gangrene, 
either in the whole or a portion of a leaf. — 
Wounds of the Stem. — The stem is very subject to wounds of various kinds, 
according to the nature of the instrument by which they are inflicted, and the 
particular texture involved. ' Wounds which penetrate or remove a portion of 
the bark, as indeed all others, heal the quicker the more vigorous the branch or 
plant is in which they occur, and of course according to their dimensions; all 
