\e?s] 



FARMERS' REGISTER. 



689 



•of serreiion is elfecipd by the combined action of 

 jfir (xt.), Light (xi.), and Temperature. 



Xf. Air and Light. 



277. When an omhr\-o plant (242.) is farmed 

 wiihin its inlPixunientP, it iis usually colorless, or 

 nearly so; but, as soon as it boixiiis to arow, that 

 part which approaches the litrhl (the stem) he- 

 comes colored, while the opposite extremity (the 

 root) remains colorless. 



278. The parts exposed to the air absorb oxy- 

 gen at niirht, absorb carbonic acid and part with 

 oxvii'en Hirain in dayliixhi; and thus in tlie daytime 

 purify the air, and render it fit for the respiration of 

 man. 



279. The intensity of this latter phenomenon is 

 in proportion to the intensity of solar light to which 

 leaves are directly exposed. 



280. Its cause is the decomposition of carbonic 

 acid, the extrication of oxygen, and the acquisi- 

 tion, by the plant, of carbon in a solid state: from 

 which, modified by the peculiar vital actions of 

 species, color, and secretions, are supposed to 

 result. 



281. For it is found that the intensity of color 

 tmd the quantity of secretions are in proportion to 

 the exposure to light and air, as is shown by the 

 deeper color of the upper side of leaves, &c. 



282. And bv the llict that if plants be grown in 

 airlrom which lijiht is excluded, neither color nor 

 secretions are formed, as is exemplified in blanch- 

 ed vegetables; which if even naturally poisonous, 

 may, from want of exposure to light, become 

 wholesome, as Celery. 



283. When any color appears in parts develop- 

 ed in the dark it is generally caused by the ab- 

 sorption of such coloring matter as pre-existed in 

 the root or other body irom which the blanched 

 shoot proceeds, as in some kinds of Rhubarb 

 when forced. 



284. Or by the deposition of coloring matter 

 formed by parts developed in light, as in the sub- 

 terranean roots, of Beet, Carrots, &c. 



285. What is true of color is also true of flavor, 

 which equally depends upon light for its existence; 

 because flavor is produced by chemical alterations 

 in the sap caused by exposure to light. (229.) 



286. The same thing occurs in regard to nutri- 

 tive matter, which in like manner is formed by the 

 exposure of leaves to light. Thus the Potato 

 when forced in dark houses contains no more am- 

 ylaceous matter than previously existed in the ori- 

 ginal tuber; but acquires it in abundance when 

 placed in the light, and deposites it in proportion as 

 it is influenced by light and air. Thus, also, if 

 Peaches are grown in wooden houses, at a dis- 

 tance from the light, they will form so little nutri- 

 tive matter as to be unable to support a crop of 

 fruit, the greater part of which will fall ofl^ And 

 for a similar reason it is only the outside shoots of 

 standard fruit trees that bear fruit. Considerations 

 of this kind form in part the basis of pruning and 

 traininsr. 



287. Light is the most powerful stimulus that 

 can be employed to excite the vital actions of 

 plants, and its energy is in proportion to its inten- 

 sity; so that the direct rays of the sun will pro- 

 duce much more powerful eflfects than the diffused 

 light of day. 



288. Hence, if buds that are very excitable are 



placed in a diffused light, their excitability will be 

 chocked. 



289. And if buds that are vt-ry torpid are ex- 

 pof^ed to direct light, ihey will be stimulated into 

 action. 



290 So that what parts of a tree shall first be- 

 gin to grow in the spring may be determined at 

 the will of the cultivator. 



291. Th.s is the key to some important practices 

 in fiircinij. 



292. This should also cause attention to be paid 

 to shading buds from the direct rays of the sun in 

 particular cases: as in thai ofcuttiniis, whose buds, 

 if too rapidly excited, might exhaust their only 

 reservoir of sap, the stem, belbre new roots were 

 formed to repair such loss. 



293. As plants derive an essential part of their 

 food from the air (280.) by the action of liirht, it 

 follows that in glass-houses those which admit the 

 greatest portion of light are the Irest adapted for 

 purposes of (uiltivation. 



294. The proportion of opaque matter in the 

 roof of a glass-house constructed of wood varies 

 trom a third to a seventh — that of an iron-house 

 does not exceed one twenty-third. 



295. Therelbre iron-rooled houses are in this 

 respect better suited for cultivation than wooden- 

 roofed houses. 



296. And it has been found by experiment, that 

 liffht passes more freely through a curvilinear 

 than through a plane roof, and through glass 

 forming an acute angle with ihe horizon than 

 through perpendicular glass; it follows that a cur- 

 vilinear roof is best, and a plane roof with glass 

 perpendicular sides the worst adapted to the pur- 

 poses of the cultivator. 



297. For the same reason common green glass 

 is less fitted for glazing forcing- houses than white 

 crown glass. 



298. Poisonous gases in very minute quantities 

 act upon vegetation with great energy. A ten- 

 thousandth part of sulphurous acid gas is quickly 

 fatal to the life of plants; and hence the danger of 

 flues heated by coal fires, and the impossibility of 

 making many species grow in the vicinity of 

 houses heated by coal fires, or in large towns. 



XII. Perspiration. 



299. It is not, however, exclusively by the ac- 

 tion of light and air that the nature of sap is alter- 

 ed. Evaporation is constantly going on during 

 the growth of a plant, and sometimes is so co- 

 pious, that an individual will perspire its own 

 weight of water in the course of 24 hours. 



300. The loss thus occasioned by the leaves is 

 supplied by crude fluid, absorbed by the roots, and 

 conveyed up the stem with great rapidit}'. 



301. The consequence of such copious perspi- 

 ration is the separation and solidification of the car- 

 bonized matter that is produced for the peculiar 

 secretions of a species. 



302. For the maintenance of a plant in health, 

 it is indispensable that the supply of fluid by the 

 roots should be continual and uninterrupted. 



303. If any thing causes perspiration to take 

 place faster than it can he counteracted by the ab- 

 sorption of fluid from the earth, plants will be dried 

 up and perish. 



304. Such causes are, destruction of spongioles, 

 an insuflicient quantity of fluid in the soil, an ex- 



