LECTURES ON BOTANY. 



237 



It may be difficult for one who has no far- 

 ther acquaintance with the act of respiration 

 than that derived from his own experience, 

 or from the casual observation oi' tlie mode in 

 which it is performed in the few animals of 

 the liigher grade with which he is accustomed 

 to associate more or less hi the ordinary occu- 

 pations of his existence, to conceive that such 

 ail act can take place otherwise than through 

 the medium of mouth and iiostidls ; but inter- 

 nal structure is as variable as external form, 

 and involves conditions of life far more diver- 

 sified than is generally miderstood. The 

 earth-worm, the spider, the bee and the but- 

 terfly are equally dependent upon an alter- 

 nate inhalation and expiration of the air they 

 move in, as aie the man, the quadi-uped, and 

 the bird ; but the process is maintained in a 

 veiy different m;uuier. The mouth in the 

 former is the vehicle tlu-ough which food is 

 conveyed into the stomach ; but they have 

 no nostrils, and breathing takes place through 

 channels in distant parts of the body. Thus, 

 in the insect, a line of pores is observable on 

 each side of the body, varying in number, but 

 frequently as many as eighteen or twenty, 

 and in certain kinds even many more. They 

 are very evident to the naked eye in many 

 of the larger caterpillars, but in the smaller 

 insects require the assistance of a good mag- 

 nifying-glass or microscope to discover them. 

 Their position is shown on one of the common 

 cabbage-caterpillars in figure 4. A feather, 



or a camel's-hair pencil, dipped in oil, and 

 drawn over these two lines of spiracles or 

 pores, kills the insect by suffocation ; and if 

 a leaf be rubbed over with oil or vaniish, so 

 as to close its stomates, it dies. Hence, in- 

 deed, one principal cause why certain plants 

 will not flourish under the shade of ti-ees, 

 while others are not affected by it. In almost 

 all of the latter class that I have examined, 

 the stomates are either wholly wanting on the 

 upper surface of the leaf, or they are compar- 

 atively ■v cry few in number. Whare tlie sto- 

 mates are numerous on the upper sirle, they 

 may become choked by the hi;avy dripping 

 from tlie leaves of the tree ui wet weather; 

 and, evaporation being checked by its shade, 

 a similar effect is liable to be produced to 

 (477) 



that arising from the coat of oil or varnish ; 

 and [ am inclined to believe that this operates 

 as injuriously, or even more so than the ob- 

 struction of light, and other causes to which 

 the evil is more generally attributed. 



To enable you to appreciate to the desired 

 extent the analogy existing between the res- 

 piration of vegetables and animals, a farther 

 examination of the structure of the leaf will 

 be necessary. By maceration, or soaking in 

 water for a few weeks, the deconqiosition of 

 the softer, cellular portion of the leaf is effect- 

 ed, the skin readily separates, and the pulpy 

 portion occupying the interstices of the veins 

 may be removed by careful washing, leaving 

 the latter entire — they being, in consequence 

 of the greater strength and i-igidity of their 

 tcxtm-e, less liable to decay than the other 

 p;u-ts. In this manner the beautiful prepara- 

 tions commonly called the skeletons of leaves 

 are obtained ; and if the maceration be con- 

 tinued a little longer, the net-work of veins 

 (at first sight appai-ently simple or consisting 

 of a single series only) divides into two, pre- 

 cisely corresponding with each other, as 

 shown in figure 5. [See next page.] 



This curious structure is perhaps more 

 readily ascertainable in the leaf of the com- 

 mon holly than in any other well-known plant, 

 the two sets of veins being less fii-mly attached 

 than in leaves of thinner substance ; but ex- 

 amples of their partial separation may be met 

 with in raoAt damp woods in the spring, among 

 the strata of dead and decaying leaves 

 that then cover the ground. Now, by 

 cutting the stalk of any leaf trans- 

 veisely, and examining the section 

 with a magnifying-glass,you may dis- 

 cern the origin of these veins in two 

 or more masses of fibre ; and by 

 caiefully cutting through the leaf- 

 stalk, and the stem or branch on 

 which it grows, lengthwise, you may, 

 with the assistance of your glass, 

 trace the connection of one portion 

 of tlie fibre — namely, that which 

 bi'auches out mto the upper net- 

 work of veins — with the woody 

 sheath that encompasses the pith: 

 the other portion, the origin of the 

 lower net- work, passes downwartl 

 into the "liber," or inner substance 

 of the bark. 



This disposition is precisely accordant with 

 the supposed functions of the leaf as an organ 

 of respiration. The sap rises through the 

 woody tubes that sunound the pith, forming 

 what is technically called the medullary 

 sheath ; and these tubes, branching out into 

 the delicate veins of the upper surliice of the 

 leaf, ex})ose it to the action of the air admitted 

 through the medium of the stomates into the 

 mtenial cavities before described. Whatever 

 may be the inture of this action, which is 

 probably chemical, the properties of the sap 

 are altogether changed by it. From a fluid 

 of comparatively simple character it becomes 

 the depository, in different vegetables, of va- 

 rious proximate principles, of the most com- 



