BOTANY. 



Leaves are generally so formed as to pre- 

 sent a large surface to the atmosphere : 

 when they are of any other hue than 

 green, they are said in botanical language 

 to be coloured. Their duration is for the 

 most part annual, but in some trees and 

 shrubs, they survive two or more seasons, 

 and such plants being always in leaf are 

 denominated evergreens. The internal 

 surface of a leaf is highly vascular and 

 pulpy, and is clothed with a cuticle very 

 various in different plants, but its pores 

 are always so constructed as to admit of 

 the requisite evaporation or absorption 

 of moisture, as well as to admit and give 

 out air. Light also acts through this cuti- 

 cle in a definite manner. That air and 

 moisture and light have considerable, and 

 even the most important effects upon the 

 leaves of plants, has long been known to 

 those who have studied the subject ; that 

 heat and cold affect them is familiar to 

 every one. The experiments of Hales, 

 Bonnet, and others, have thrown much 

 light upon the absorption and perspira- 

 tion of leaves, while those of Priestley 

 and Ingenhouz have explained their ef- 

 fects upon the atmosphere, and the man- 

 ner in which air and light particularly act 

 upon them. Leaves have a natural ten- 

 dency to present their upper surface to 

 the light, and turn that surface towards 

 it, in whatever direction it is presented to 

 them. When trees in leaf are nailed to a 

 wall, and the position of their leaves is 

 consequently disturbed, they soon reco- 

 ver their natural direction. Light evi- 

 dently acts as a wholesome stimulus to 

 their upper surfaces, and as a hurtful one 

 to the under. When the latter is forcibly 

 presented for a long period to its rays, 

 destruction is the consequence. Leaves 

 seem to require occasional repose from 

 the action of light on their upper surface ; 

 for, when it is withdrawn from them, ma- 

 ny leaves close or fold themselves toge- 

 ther,as if in a state of relaxation, and spread 

 themselves forth again at the returning 

 beams of the morning. This is more es- 

 pecially the case with winged leaves, as 

 those of the pea kind. Those of the 

 white acacia, robinia, pseudo-acacia, have 

 been remarked by Bonnet to be over- 

 excited by the sun of a very hot day, and to 

 fold their upper sides together, in a man- 

 ner directly contrary to their nocturnal 

 posture. The effect of moisture upon 

 leaves every one must have observed. 

 By absorption from the atmosphere, they 

 are refreshed, and by evaporation, espe 

 cially when separated from their stalks, 

 they soon fade and wither. Aquatic ve- 

 getables, whose leaves are immersed in 



the water, both absorb and perspire with 

 peculiar taciliiv. Anatomical investiga- 

 tions have shewn thai the nutricious 

 juices, imbibed from the earth, and be- 

 come sap, are carried by appropriate ves- 

 sels into the substance of the leaves. Mr. 

 Knight, in his papers in the Philosophi- 

 cal Transactions, has demonstrated that 

 these juices are returned from each It af, 

 not into the wood again, but into the 

 bark. Hence a new and curious theory 

 of vegetation has been established. It 

 appears that the sap is carried into the 

 leaves for the purpose of being acted up- 

 on by air and light, with the assistance 

 of heat and moisture. By all these agents 

 a most material change is wrought in its 

 component parts and qualities, differing 

 widely according to the diversity of the 

 species. Thus the resinous, oily, mucila- 

 ginous, saccharine, bitter, acid, or alka- 

 line secretions are elaborated. The heed- 

 less observer of a leaf is little aware of 

 the wonderful operations constantly going 

 on in its delicate substance, nor can the 

 most enlightened philosopher explain 

 more than a very small part of the chemi- 

 cal processes of which it is the immediate 

 agent. It is scarcely necessary to ob- 

 serve how materially plants differ in the 

 flavour and qualities of their leaves, all 

 which must depend in a great measure 

 on the operation of the leaf itself, for the 

 common sap of plants, from which all 

 their secretions are made, differs very 

 little in plants whose qualities are very 

 unlike to eacli other ; those qualities de- 

 pending upon the secreted fluids elabo- 

 rated principally by the leaves. 



The green colour of the organs in 

 question is easily proved to be almost 

 entirely owing to the action of light. 

 Plants which grow in the dark are of a 

 sickly white, which is the case with any 

 parts artificially or accidentally covered 

 with earth, as in cultivated cellery or as- 

 paragus, whose stems and leaf-stalks are 

 purposely managed in this way, to render 

 their flavour and appearance more deli- 

 cate. Such blanched parts soon become 

 green on exposure to light. Leaves are 

 subject to a sort of disease, by which they 

 become partially spotted or streaked with 

 white or yellow. In this state they are 

 termed variegated, and occasionally con- 

 tribute to the ornament of our gardens. 

 The whiteness frequently extends to the 

 leaf-stalk, and sometimes to the branch, 

 as may be seen in the variegated alder. 

 Such varieties are propagated by cut- 

 tings, layers, or roots, but not by seed. 

 They appear to be somewhat more ten- 

 der than the plant in its natural state. 



