AN ENCYCLOPADIA 
OF HORTICULTURE. 115 
Hawthorn Caterpillars—continued. 
They are smooth, have a row of dark spots on the sides, 
and the head is brown. The nests are often extremely 
conspicuous. The Sawflies that do most harm to Haw- 
thorn are: Dineura stilata, Eriocampa limacina (E. adum- 
brata), .and Lyda punctata. The larve of Lyda want 
prolegs, and feed in a web, spun over the branches. 
Each larva also makes a special silken case for itself 
(see Lyda) The larvæ of E. limacina eat away the 
upper surface of the leaves, often stripping it entirely 
off; and the leaves, in consequence, become marked with 
brown, scorched spots, or, maybe, completely killed. 
Frequently, great injury is done by these attacks. The 
larvæ usually feed in groups of three or four. They 
resemble small greenish-yellow slugs, covered with a 
slimy secretion, and are commonly known as “ Slug- 
Fic. 171. SMALL ERMINE MOTH AND WEB OF CATERPILLARS. 
worm.” Several cultivated trees are subject to their 
attacks. Dinewra stilata is very similar to the last 
Species in the mode in which the larve feed; but the 
latter are uniformly green, have the legs quite visible 
while on the leaves, and they emit a disagreeable smell. 
Further information will be given under Sawflies 
(which see). The best means of prevention, or of cure, 
in respect to these insects are as follows: Larvae living 
Socially in webs are easily removed and destroyed with 
the webs. The larvæ on the exposed leaves are readily 
killed by dressing the plants with the powder of Helle- 
bore in water, or by using Paris Green. The foliage of 
Hawthorn is frequently mined by the larve of various 
small insects, chiefly moths, but the plants do not suffer 
much real injury in this way. 
HAZEL. See Corylus Avellana. 
HAZEL, WITCH. ‘See Hamamelis. 
HEADING, or HEARTING. A term applied to 
various members of the Cabbage tribe, when their central 
leaves cease to unfold and commence forming what is 
usually known as the heart. The closeness of the latter 
is regulated by exposure to light. Summer is favourable 
to Heading, even when the plants are in a young state, 
and winter timo against it. 
HEADING-DOWN. This applies to the severe prun- 
ing of trees and shrubs that ‘have become overgrown. 
In some cases, it is practised for the encouragement of 
a better and cleaner growth when signs of declining 
vigour are apparent. Heading-down will be requisite 
with fruit-trees which it is intended to graft by any of the 
methods usually employed, except inarching. - 
HEARTSEASE. See Viola tricolor. 
HEARTWOOD, The central part of the timber of 
exogens, hardened or altered by age. 
HEATH. See Erica. 
HEATHER. Sce Calluna vulgaris. i 
HEATH, ST. DABEOC’S. See Dabecia poli- 
HEATH, SEA. See Frankenia. ; 
HEATHWORTS. A name given by Lindley to the. 
order Ericaceæ. 
HEATING. This, in connection with horticultural 
structures, is an absolute necessity for securing and regu- 
lating temperatures artificially, to suit the requirements 
of exotic plants; and for the production of flowers, fruits, 
and vegetables out of their natural season. Its effects 
may be derived from fermenting material placed inside 
the structure, or from causes which arise as the product 
of combustion by fire in the immediate vicinity, trans- 
mitted, by means of water or air, to wherever it is de- 
sired. These sources of heat, either used separately or 
in combination, afford the requisite temperatures for 
different plants, according as their admission is regu- 
lated to the various houses in which the latter are 
grown. Fermenting material evolves a considerable 
amount of heat, but by a slower process than combus- 
tion, as usually understood. A more genial and moist 
temperature may be secured from the former than 
from fire heat, but it cannot be so readily regulated. 
A fermenting mixture of litter and leaves greatly 
encourages thé growth of young plants in spring, 
and is also preferable for starting early Vines and 
fruit-trees. It is advisable, in case of severe weather; 
to make provision for adding fire heat as well. Gentle 
hotbeds are also very useful for forcing vegetables, 
and for the raising of seeds generally. Heating by 
hot air is not adapted for horticultural purposes, on 
account of the consequent drying of the atmosphere 
being very injurious to plant life. Flues are but little 
better; still, means may be adopted for moistening the 
heated air transmitted by them, where it is imprac- 
ticable with a continued influx of dry air. Both of 
these systems may, therefore, be dismissed in reference 
to all glass houses of modern construction, and one of 
the varions methods of Heating by hot water should 
be, in all cases, adopted. Before proceeding to notice 
some of the most approved boilers for requirements on 
a large or small scale, it may be well to refer to the 
principles applied to Heating, as on these being properly 
understood and carried ont in the construction of any 
hot-water apparatus, success or failure in its action ma- 
terially depends. Heat always has a tendency to equalise 
itself, by communicating part of its properties to, sur- 
rounding substances until they are raised to an equal 
temperature, so far as the original intensity admits. If 
