AN ENCYCLOPAEDIA 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 

 conspicnons. The Sawflies that do most harm to Haw- 

 thorn are : Dineura stilata, Eriocampa limacina (E. adum- 

 brata), and Lyda punctata. The larvae 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 larvae of E. limacina eat away the 

 npper 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 

 larvaa 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- 



FIG. 171. SMALL ERMINE MOTH AND WEB OF CATERPILLARS. 



worm." Several cultivated trees are subject to their 

 attacks. Dineura stilata is very similar to the last 

 species in the mode in which the larvaa 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 larvae 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 larvaa of various 

 Email insects, chiefly moths, but the plants do not suffer 

 much real injury in this way. 



See Corylns 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 time 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. See Calluiia vulgaris. 



HEATH, ST. DABEOC'S. See Dabcecia poli- 

 folia. 



HEATH, SEA. See Prankenia. 



HEATHWORTS. A name given by Lindley to the 

 order Ericaceae. 



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 the 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, 

 j 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 

 j to all glass houses of modern construction, and one of 

 the various methods of Heating by hot water should 

 j be, in all cases, adopted. Before proceeding to notice 

 i some of the most approved boilers for requirements on 

 j 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 out 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 

 1 temperature, so far as the original intensity admits. If 



