236 



MONTHLY JOURNAL OF AGRICULTURE. 



important function of respiration — a fiinction 

 upon which seem primarily to depend those 

 modifications of the sap that are essential to 

 the production of the secretions that give so 

 great a diversity of character to different veg- 

 etables, and render them available to the va- 

 rious purposes of human economy. Consid- 

 erable analogy, too, unquestionably exists be- 

 tween the breathmg apparatus of plants and 

 aniuials. In the higher orders of the latter 

 the lungs are almost wholly composed of the 

 ramifications of the bronchial tubes or branch- 

 ings of the windpipe, and two sets of blood- 

 vessels, called the pulmonary veins and arte- 

 ries — the ultimate divisions of the three being 

 so minute and intricately blended as almost 

 to baffle the researches of the anatomist, even 

 though assisted by the highest powers of the 

 microscope. The delicate extremities of the 

 air-tubes terminate each in a little rounded 

 cell, over the walls of which branch the al- 

 most inconceivably miiuite extremities of the 

 arteries and veins — the blood circulating in 

 which is thus brought into close contact with 

 the air drawn into the lungs at each mspira- 

 tion. 



It would be foreign to our subject to enter 

 into description or discussion of the physiolo- 

 gical phenomena connected with tins function 

 in the animal, but it is one apparently essen- 

 tial to all orgaiuc beings, however it may be 

 modilied in their several grades to accoid 

 with their greater or lesser complexity of 

 structure, and their diversified habits and 

 modes of existence. Let us now examine the 

 leaves, and endeavor to ascertain how far the 

 received opinions concerning their respiratory 

 functions are capable of being substantiated 

 by fact. 



Viewed apart from the plant on which it 

 grows, the leaf is a very curious and complex 

 structure. A frame of fibre branching fi-om 

 the top of its stalk, or, v^diere the stalk is 

 wanting, from the base of the leaf itself, form- 

 ing a kind of net-work, the interstices of 

 which are fitted up with a green, soft sub- 

 stance, the whole inclosed within a thin skin 

 or membrane, is all perhaps that the unas- 

 sisted eye is cajiable of detecting; but the 

 fibrous frame-work, when magnified, is dis- 

 covered to consist of the woody and spiral 

 tubes already described, compaclly bound to- 

 gether at the lower part, and gradually sepa- 

 rating from each other in the form of veins as 

 they extend toward the margins and extrem- 

 ity. The green, pulpy matter occupying the 

 intervals between their ramifications consists 

 of cellular tissue, filled with green particles — 

 its little cells not contiguous throughout, but 

 leaving small open sfjaces here and there ; 

 while the skin-like covering of the leaf (epi- 

 dermis) consists ofa layer of small compressed 

 cells, apparently empty or only filled with 

 air, colorless and transparent, so as to admit 

 of the colored veins and green tissue of the 

 interior l)eing visible through them. The 

 skin or epidermis varies in texture in the 

 leaves of different plants : as does, likewise, 

 (476) 



frequently that of the upper and under sur- 

 faces of the same leaf The size and arrange- 

 ment of the cellules of which it is composed 

 are often very irregular. It possesses consid- 

 erable rigidity and toughness in some plants, 

 and in all serves to protect the more delicate 

 tissue beneath from external injury. In the 

 greater number of vegetables its continuity is 

 interrupted by pores or openings, technically 

 denominated "stomata," which occur in some 

 cases only on the imder surface of the leaf, in 

 others on both sides, and are more or less nu- 

 merous. Thus, in the leaf of the common 

 lilac, which has none on the upper face, 160,- 

 000 have been counted on one stprare inch of 

 the lower ; while in that of the carnation, 

 within the same space, there are 38,500 on 

 each side. The stomates are not mere perfo- 

 rations in the epidermis, but spaces, generally 

 of an oval form, in the middle of which is a 

 slit that opens or closes according to the con- 

 dition of the atmosphere, or other circum- 

 stances affecting the state of the growing 

 plant. The oval border consists, in most in- 

 stances, of two oblong parallel cells, capable 

 of contracting, so as to become somewhat kid- 

 ney-shaped, thus opening the stomata ; which, 

 so long as they reinaui straight, are closed. 



Figure .3 represents a portion of the cuticle 

 of the elk's-horn fern ( acrostichwm alcicorne) 

 highly magnified, with several of its stomata ; 

 and at S is a vertical section of one of them, 

 showing the vacant spaces into which it opens 

 in the substance of the leaf. 



Between lliese stomates and the spiracles 

 or breathing-pores of some of the lower orders 

 of animals, considerable resemblance unques- 

 tionably exicits ; and numerous experiments 

 (of which our space will not admit any detail) 

 have confinned the opinions entertained by 

 physiologists concerning their corresponding 

 functions. 



