SECRETION. 



443 



or from the soil, and combining this with the 

 other three elements into quaternary com- 

 pounds, that seem destined rather for the 

 nutrition of animals than for any special pur- 

 pose in the economy of the plant itself. The 

 contents of the cells of the leaves are thus of 

 a very complex nature ; their life not be- 

 ginning and ending with themselves, as is the 

 case with that of the independent organisms, 

 which in other respects they resemble ; but 

 having relations to the rest of the structure, 

 for which, in fact, it is their function to pre- 

 pare the pabulum. For the elaborated sap or 

 nutritious fluid, which is the product of their 

 agency, is transmitted through the entire 

 fabric, and furnishes each portion with the 

 materials of its development and extension, 

 which in every instance is effected by an act 

 of cell-growth. All parts select from it the 

 same substance for the formation of the cell- 

 walls, but the cell-contents are different in 

 every organ and variety of tissue. Thus we 

 find one set of cells drawing in starch, another 

 fixed oil, another resin, another volatile oil, 

 another colouring matter, another sclerogen, 

 another protein compounds, and so on ; and 

 this with the greatest uniformity and regu- 

 larity. We may frequently see that even 

 contiguous, and in other respects similar, 

 cells, in the same organ, either select from 

 the common pabulum a different compound, 

 or exercise upon the same compound a dif- 

 ferent influence. Thus we observe in the parti- 

 coloured petal of a hearts-ease or tulip, certain 

 stripes or patches of different hues, which, 

 when examined with the microscope, are 

 found to consist of cells that differ from each 

 other only in the colour of their contents. A 

 precisely similar phenomenon is presented by 

 the epidermic cells, which constitute the 

 scales of the wings of Lepidoptera. 



In all these cases, however, the products 

 which are separated from the circulating 

 fluids are stored up within the component 

 cells of the fabric, instead of being cast forth 

 from it ; and although the term secretion is 

 commonly applied to the process, yet it would 

 be just as correct to regard it as part of the 

 function of nutrition. It is, in fact, exactly 

 on the same footing with the production of 

 fat in animals. 



The absence of necessity for any other 

 form of excretion in plants, than that which 

 is carried on through the respiratory process, 

 may be accounted for without much difficulty. 

 A large proportion of the vegetable fabric is 

 (from the nature of its chemical constitution) 

 but little prone to decomposition, and pos- 

 sesses a character so permanent, that it may 

 remain almost unchanged for an indefinite 

 time; and those parts which are of softer 

 texture and more actively employed in the 

 vital processes, and which are therefore more 

 prone to decay, are periodically thrown off 

 and renewed. In animals, on the other hand, 

 all the softer tissues have a strong tendency 

 to disintegration, in virtue of their peculiar 

 composition ; and in some of them a destruc- 

 tive chemical change seems to be the very 



condition of their functional activity. For 

 the maintenance of their vital energy, there- 

 fore, there is needed not merely a constant 

 supply of new material, but a continual re- 

 moval of the effete particles. On this last 

 operation, indeed, the continuance of the vital 

 activity of animals is more closely and imme- 

 diately dependent, than it is upon the supply 

 of aliment ; for whilst the latter may be in- 

 terrupted for a period of considerable dura- 

 tion without producing more than debility, 

 the former cannot be checked for many hours 

 (in the warm-blooded animals at least) with- 

 out a fatal result. Indeed, if we consider 

 respiration as one of the excreting processes 

 (which it undoubtedly is in a broad and 

 philosophical acceptation of the latter term), 

 we must say that the liberation of effete 

 particles may not be suspended for more than 

 a few minutes without death ensuing. 



Turning our attention, then, in the first 

 instance, to the excretory organs of animals, 

 we may define them to be groups of cells, 

 placed on the free surface of a membrane, 

 which is directly continuous with that of the 

 exterior of the body, whilst its attached sur- 

 face is in relation to the blood-vessels, &c. of 

 the interior ; so that these cells, having grown 

 and developed themselves at the expense of 

 the materials supplied by the bloo.l, are either 

 cast off entire and conveyed away, or give up 

 their contents by the rupture or deliquescence 

 of their walls ; the products which they have 

 selected or eliminated being thus, in either 

 case, entirely got rid of from the interior of 

 the fabric. The disposition of the membrane 

 on which the cells lie, whether it be spread 

 out on a plane surface, depressed into short 

 rounded follicles, or extended into long and 

 convoluted tubes, is a matter of secondary 

 consequence ; nor is it of more importance 

 whether the follicles be isolated, and discharge 

 their contents by separate outlets, as those of 

 the skin or mucous membrane (fig. 307.), or 

 whether they are aggregated in clusters, and 



Fig. 307. (Fig. 209. Vol. II.) 



Glandular follicles in ventrirultm succenturiatus of 

 Falcon and other birds. (After JUilller.') 



