526 



THE DICTIONARY OF GARDENING, 



Sun-burning 1 continued. 



the result; but it is impossible to determine the share 

 due to each. A very rapid rise of temperature, after 

 severe cold, is also apt to produce results included 

 under the general name of Sun-burning. 



Most plants, if exposed for some time to a temperature 

 between 105deg. and 125deg. Fahr., are killed ; but cer- 

 tain fleshy subjects, particularly species from tropical 

 deserts, are able to resist even a higher temperature 

 than 125deg. When plants that have been in a green- 

 house all the winter are first put out of doors, in spring, 

 the leaves often become brown and look scorched. These 

 organs may die and wither, or may only become red or 

 brown, and afterwards reassume their natural green 

 colour. Plants do not usually suffer serious injury from 

 this cause, though often checked in their growth for 

 a time. 



In ill-ventilated glasshouses, it often occurs that the 

 leaves of many of the plants show round, pale, withered 

 spots. Observation teaches that these follow the pre- 

 sence on the leaves, during sunshine, of drops of water ; 

 and it has been suggested that the drops act like minia- 

 ture burning-glasses, focussing the rays upon the spots 

 below them, and destroying the protoplasm by the excess 

 of light and heat so caused. Inequalities in the glass 

 of greenhouses are believed to give rise to similar 

 injuries. Whatever may be the cause, experience has 

 proved that thorough ventilation is the best preventive, 

 and that, if this is attended to, the evil will be much 

 diminished, if not wholly cured. 



Sun-burning and Sunstroke are terms applied to in- 

 juries of the stems of trees, consisting sometimes in the 

 bark dying, and separating from considerable surfaces, 

 or in long strips, on the side most exposed to the sun's 

 rays. Sunstroke is apt to occur in trees exposed by the 

 removal of others or of undergrowth, or it may follow 

 the erection of walls or other reflectors of heat in the 

 immediate neighbourhood of the stems. It injures both 

 fruit-trees (especially Peaches) and forest-trees when 

 suddenly exposed. The cause of injury is the death of 

 the newly-formed cells of the cambium layer by exposure 

 to excessive heat. In trees suddenly exposed to varia- 

 tions in temperature greater than they had previously to 

 bear, the bark is not sufficiently developed to protect 

 the cambium from injury, and the result is as just de- 

 scribed. 



Splitting of the bark is apt to occur in spring, when 

 trees have been exposed for some time to keen frost, 

 followed by a sudden rise of temperature. The injury 

 is due to unequal expansion of the bark and the wood. 

 Probably, it is more often caused by the preceding frost 

 than by the warmth, the thaw only bringing to view the 

 harm already done. But that Splitting is, to some 

 extent, connected with the sun's warmth is shown by 

 its greater frequency on the side of the trunk that 

 receives most of the sun's rays. Both Sun-burning and 

 Splitting are far commoner on the Continent than in 

 Britain, where the extremes of heat and cold are 

 seldom such as to give rise to either. The best pre- 

 ventive of both is to protect the trunks and branches 

 that may be in danger, by some simple means of shelter, 

 such as wrapping them in straw, or coiling a straw-rope 

 around them. 



A few years ago, Dr. H. Muller, of Thurgau, called 

 attention to the loss of grapes that occurs in various 

 places in Germany when cold, damp weather is sud- 

 denly followed by bright, warm, sunny days. In the 

 unripe clusters that are exposed to the direct rays of 

 the sun, many of the grapes become pale, and then 

 shrivel and turn brown. Sometimes, the stalks of the 

 clusters turn brown before the grapes show signs of 

 injury, but the latter soon shrivel and die when the 

 stalks are killed. Experiments show that the cause of 

 injury is excessive heat, and that similar results follow 



Sun-burning continued. 



when clusters are exposed to artificial heat as great as 

 that to which they are subjected under natural condi- 

 tions, i.e., between 105deg. and 115deg. Fahr. The risk 

 of injury is greater in proportion as the fruit is more 

 juicy. The moister the atmosphere, the less must be the 

 evaporation from the fruit ; hence, in moist weather, 

 evaporation cannot act as a means of keeping down the 

 temperature, and this probably explains how the harm 

 done is greater after a continuance of wet weather, as 

 the air is then nearly saturated with moisture. The risk 

 is much less in England than in Germany, as the tem- 

 perature is seldom so high in our islands as to do injury. 

 Prevention may be secured by any method of shading 

 the clusters. The best protection is that afforded by the 

 leaves of the Vines themselves; hence, the removal of 

 the leaves to hasten ripening is seldom to be recom- 



Though not strictly included under Sun-burning, a 

 few remarks may be added on the harm done by dry, 

 windy weather to many plants that grow in moist soil. 

 If exposed for some days to dry, warm winds, the leaves 

 of such plants wither and die, becoming so brittle as to 

 crumble into dust when rubbed in the hand. Under 

 similar conditions, the same species of plants growing in 

 drier ground may scarcely be injured. The cause is as 

 follows : The damper the localities in which plants grow, 

 the more are all their green parts adapted for rapid 

 evaporation. While the evaporation is balanced by the 

 amount of water absorbed by the roots, and carried up 

 by the stems, growth goes on rapidly, and the plants 

 grow luxuriantly, unless the mineral matters absorbed 

 with the water accumulate to a hurtful degree, when 

 the plant will become weak and sickly. But in dry, 

 warm weather, the loss by evaporation exceeds the 

 amount that can be supplied to the leaves; hence, they 

 wither, and dry up. Those that suffer most are the 

 mature leaves in the active discharge of their functions, 

 the older and younger ones being far less severely 

 injured. The only remedial means applicable is to 

 shelter the choicer plants from wind and sunshine as 

 much as possible, and to syringe their leaves occasion- 

 ally. Watering the soil is of no use, as it is already 

 too moist. On such ground, drainage is the most 

 efficient means of preventing injury to the plants from 

 drought. 



SUNDEW. See Drosera. 



SUNDROFS. A name applied to (Enothera fruti- 

 cosa. 



SUNFLOWER. A popular name for the species 

 and varieties of Helianthus, but more particularly applied 

 to the varieties of H. annuus, which may readily be 

 raised from seed each year. Sunflowers are best adapted 

 for planting at the back of large shrubbery borders 

 or in wild gardens. 



SUNIPIA (said to be the native name in Nepanl). 

 OBD. Orchidece. A monotypic genus. The species a 

 stove, epiphytal Orchid, with small, racemose flowers, 

 lateral, elongated, leafless scapes, and a coriaceous, slen- 

 der, many-veined leaf is a native of Java, and awaits 

 introduction to this country. 



SUN-PIiANT. A popular name for Portulaca grandi' 

 flora and other species. 



SUN ROSE. See Helianthenium. 



SUPERIOR. Growing above anything. The pos- 

 terior or upper lip of a corolla is the Superior. " A calyx 

 is Half-superior when it appears to grow from above 

 the base of an ovary, and absolutely Superior when it 

 appears to grow from the top of the ovary. On the 

 contrary, the ovary is Superior when it grows above 

 the origin of the calyx " (Lindley). 



