172 



FARxMERS' REGISTER— VEGETABLE PHYSIOLOGY. 



to counterbalance the quantity introduced into the 

 atnfiosphere by other causes. In this single func- 

 tion, then, of vegetable life, we see an eiricient 

 yet simple means of restoring to the atmosphere 

 that proportion of oxygen which is necessary to 

 the health and existence of organized beings. — 

 Thus, the consideration of the humble functions of 

 vegetable life may serve to elevate our thoughts 

 in wonder at that universal order which prevails 

 throughout the works of the gi-eat creator and pre- 

 server of all things. 



Fifth period of nutrition. — This period com- 

 prises the return of the newly elaliorated sap into 

 the system. The course which it pursues in its 

 altered condition is, for the most part, down the 

 innermost layers of the bark ; as several very satis- 

 factory experiments have decided. In tlie pro- 

 cess of " ringing," which consists in stripping a 

 branch of a circular rim of bark, the descending 

 sap Is stopped, and a swelling is formed at the su- 

 perior edge of the ring. Tlie alburnum, or soft 

 wood immediately below the bark, will also, if 

 properly protected, servs to convey a portion of 

 the descending sap, which is then further elabo- 

 rated, and tlie alburnum hardens to solid wood. — 

 It has been a subject of considerable controversy, 

 whether the new layers of wood and bark have re- 

 sulted from the developement of the old tissues 

 nourished by the descending sap, or whether they 

 have been generated partly in this way, and are 

 partly formed of fibres descending from the buds 

 which are seated on all parts of the stems and 

 branches. Perhaps the question ought not to be 

 considered as finally settled ; but certainly we 

 agree with our author in considering the latter opi- 

 nion, of the descent of the fibres, as one v.'hich is 

 little supported by facts, or by analogy, and that 

 it rests for the present entirely upon vague con- 

 jecture and hypothetical reasoning. The elabo- 

 rated sap descends to the roots, and causes their 

 further developement, whilst a portion of it is in- 

 tercepted by the rising sap by which it is convey- 

 ed to the buds on the stem. There is, in fact, no 

 true circulation in plants ; but a portion of the 

 elaborated sap, in its descent towards the roots, be- 

 comes mixed with the ascending sap, and is thus 

 conveyed to all parts of the system. 



Sixth periodof nutrition. — In detailing the fourth 

 period, we accounted for one alteration in the con- 

 dition of the sap, by the circumstance of a conside- 

 rable exhalation of its aqueous particles taking 

 place, and for another, by its having received an 

 addition of carbon from the decomposition of car- 

 bonic acid in the green parts. The nutritious ma- 

 terial thus formed is essentially composed of car- 

 bon, and the two elementary ingredients of water, 

 viz. oxygen and hydrogen. There are, however, 

 several vegetable products, differing materially 

 in their sensible qualities, which are composed of 

 these three elementary substances only, and it is a 

 task of some delicacy to select that particular one 

 from among them, which may most reasonably be 

 considered as the universal pabulum prepared for 

 the nourishment of the different vegetable tissues. 

 Our author considers " gum" to be the simplest 

 combination of the three elementary ingredients 

 mentioned ; and argues, from its universal preva- 

 lence, that it must be the true nutritious principle 

 of vegetables. There are some other substances 

 nearly allied to gum in their chemical composition, 

 which appear to be slight modifications of it, cf- | 



fected in some after process, by the secreting pow- 

 ers of the cellular tissue. The preparation of 

 these fresh substances constitutes our sixth period. 

 They seem to serve some purpose connected with 

 the nutrition of the plant ; but what this may be, 

 it is impossible in the present state of our know- 

 ledge, to decide. Three of these substances are 

 fecula, sugar, and lignine, each differing very lit- 

 tle from gum in their chemical composition. If 

 we consider the solution of gum, so constantly 

 ibund in the sap of plants, as analogous to blood, 

 the formation of these other materials may then 

 be likened to certain local secretions in the ani- 

 mal kingdom. Each gi-ain of " fecula" appears 

 to be a reservoir of gimi incased in an insoluble 

 integument. It is diffused through various parts 

 of the plant, serving as so many magazines of nu- 

 triment for tlie future developement of its several 

 organs. It bears a striking analogy to the fat of 

 animals. "Sugar" bears a strong resemblance 

 both to gum and fecula in its composition. It is 

 ibund in a liquid state in the cells, and is j^robably 

 intended to serve some purpose or other of nutri- 

 tion. " Lignine" is insoluble in water, and is a 

 secretion deposited in the cells, which compose the 

 woody portions of the plant. 



Seventh period of nutrition. — The three sub- 

 stances mentioned under the sixth period, appear 

 to be destined to serve some purpose or other of 

 nutrition, as well as that universal pabulum "gum," 

 of which they are only slight modifications. Be- 

 sides these, there are many other substances 

 vvhich result from the specific action of distinct 

 parts of the vegetable structure, andAvhich bear a 

 still closer analogy to the peculiar products secre- 

 ted from the blood of animals by the action of par- 

 ticular glands. In animals, however, the glands 

 destined for this purpose are very conspicuous, the 

 ducts through which the secreted matter is con- 

 veyed being clearly defined, and the secretions 

 themselves presented to us in an isolated form. — 

 But in plants, the glands are generally minute, 

 their structure scarcely distinguishaL)lc, and many 

 of tfieir secretions so much blended with other ma- 

 terials, that it requires a chemical process to sepa- 

 rate them. 



The enumeration of these various substances oc- 

 cupies a considerable portion of the first volume ; 

 and an attempt is made to classify them under a few 

 general heads ; but as no light is thus thrown upon 

 the function of secretion, we shall allude very 

 briefly to these details. Every separate vesicle of 

 the cellular tissue may be considered as a secre- 

 ting organ, and some of them appear to elaborate 

 peculiar compounds without assuming any of the 

 ordinary characters of disiinct glands. In other 

 cases a glandular structure is clearly distinguisha- 

 ble from the rest of the tissue. This matter elabo- 

 rated is either destined to appear on the outside as 

 an excretion, or it remains within the plant, but is 

 so arranged in separate cavities as not to intermix 

 with the nutritive juices. These all differ from 

 those other secretions which we have considered 

 as eminently nutritive, by having their oxygen 

 and hydrogen in a different proportion from that 

 in which they exist in water ; and hence it seems 

 probable that they result from a later and more 

 complicated process than that which produces the 

 nutritive secretions belonging to the former pe- 

 riod. They all moreover act as poisons when 

 imbibed by the roots; and this again shows uSj 



