May I, 1884.] 



THE TROPICAL AGRICULTURIST. 



775 



YELLO\VTXG OF LEAV:ES. 



A yellow coloration of the leaves is too sure a sign of 

 want of vigour in a plant, and is often riglitly enongli 

 attributed to some mismanagement upon the part of the 

 gardener ; but, although this truth is generally recognised, 

 it is not so well known to what precise causes the sickly 

 condition is due. Some recent experiments of M. A. Le- 

 clerc, performed, as it would appear, with all the attention 

 to details, and all the care requisite to eliminate possible 

 som'ces of compUcation and error, tend to throw consider- 

 able light upon the matter; and, although Mr. Leclerc's 

 conclusions are based to a large extent upon plants grown 

 in the open field by the farmer, yet it is clear that, if 

 well founded, they must apply equally well, and indeed 

 in many respects more definitely, to plants grown under 

 the more neatly-defined conditions imder which a gardener 

 places his plants. ]\I. Leclerc's experiments in the first in- 

 stance were purely laboratory experiments, undertaken with 

 a view of ascertaining the amount of watery vapour 

 exhaled by plants, and the conditions under which that 

 process takes place, but he was afterwards led to apply 

 the conclusions at which ho had arrived in the laboratory, 

 where conflicting elements were eliminated, to the rougher 

 test of the field, where all sorts of complications less under 

 control of the cultivator arise. 



Experiments showing the vast amount of fluid lost in the 

 form of vapour or otherwise, have long been familiar to 

 physiologists in this country from the time of Hales to 

 that of Lawes and Gilbert, not to mention experimenters 

 of other nations. "We need not revert to these, nor to the 

 confirmatory experiments of M. Leclerc. It may sufBce to 

 give some of the main conclusions aiTived at by the last- 

 named observer as detailed in a recent number of the 

 Annalc?! des Sciences NaUirelles. As the results of his delic- 

 ate and precise experiments, SI. Leclerc shows that light 

 by itself has no iudueuce on transpii-ation, and that, as 

 might have been expected, it ceases when the atmosphere 

 is saturated mth moist vapour. At first sight it would 

 seem as if a plant exposed to the light must necessarily 

 evaporate more fluid from its surface than one not so ex- 

 posed, and that is so ; but the effect is not due to the 

 light only, but partly to the heat-rays which act conjointly 

 with the light, and by heating the tissues cause them to 

 give up their vapour, and partly to the chemical action 

 which goes on in the leaves during the process of assimil- 

 ating the carbon from the carbonic acid gas, and the con- 

 sequent elimination of oxygen — functions which, as is well 

 known, are exercised by all the green parts of plants when 

 exposed to solar or electric light. 



Turning to matters of more direct practical importance, 

 M. Leclerc proceeds to speak of the yellow appearance so 

 familiar to \Vheat growers in certain states of the atmo- 

 sphere. In his numerous experiments on evaporation M. 

 Leclerc always found that plants grown for a certain*time 

 in a saturated atmosphere, or one which was near satur- 

 ation point, soon became yellow. This yellowing was evid- 

 ently caused by the atmosphere being overcharged with 

 moisture. Previously he had considered that effect as the 

 nonsequence of excess of water in the soil; but on examin- 

 ing into the validity of these two opinions, so radically 

 different, and by the aid of easily repeated experiments, 

 he arrived at the conclusion that the cause of the yellowing 

 was intimately connected with transpiration, and con- 

 sequently was dependent on the state of the atmosphere. 

 By means of an experiment, which we need not describe 

 in detail, but the object of which was to expose the foliage 

 alternately to dry and saturated atmospheres, the other 

 conditions being uniform, and the roots in all cases plunged 

 in water, it was fouml that the plants whose foliage was 

 exposed to a saturated atmosphere became yellow. The 

 yellow plants were then placed in a dry atmosphere, and 

 the green ones in a satm-ated atmosphere, thus reversing 

 the previous experiment. The result was that the yellow 

 leaves became green, and the green ones yellow. A repet- 

 ition of the experiments in both directions produced the 

 same results. Thus atmospheric conditions alone sufficed 

 to bring about these changes, the roots remaining in all 

 cases under the same conditions. These results were tested 

 in various ways, but the same conclusions were always 

 arrived at — viz., that, the yellow state of the leaves was 

 the result of diminished transpiration, due to the more or 

 less complete saturation of the atmosphere with moisture. 



In other cases an imhealthy condition is brought about 

 not by diminished, but by excessive transpiration. For in- 

 stance, if a plant be grown under an almost constantly 

 clouded sky, the atmosphere and soil alike charged with 

 moisture, and then these conditions be suddenly altered, 

 so that the leaves become exposed to a bm-uing sun, then 

 evaporation becomes excessive and the leaves wither. Uy 

 removing a certain number of the leaves, and thus reduc- 

 ing the amount of evaporation, the ill-effects were checked. 

 By observing what happened under given conditions, and 

 by noting the result of reversing and alternating those 

 conditions, it was shown conclusively that the unhealthy 

 conditions were due in the one case to defective, in the 

 other to excessive, transpiration. The gardener has the 

 advantage over the farmer in this particular, that he can 

 to a large extent regulate matters and prevent either ex- 

 treme. For this nothing more than careful observation and 

 tact, born of experience, are required. — Gardeners^ Chronicle. 



SAP IN TE^V. 



A question of the utmost importance to those interested 

 in our tea gardens is beginning to crop up, which, though 

 occasionally pooh-poohed, will have to be answered some 

 day, and perhajjs at no distant date. The question is 

 whether, as the plants grow okier and the old stems in 

 consequence become tough and gnarletl, the sap does not, 

 in i^enetrating the tissues, lose a great deal more of its 

 vigor than when rising through the stems of the new 

 plants ; and, in consequence, is there any soimdness in the 

 assertion that the teas from our old gardens show a 

 marked deterioration in strength and flavor as conip;ired 

 with the produce of young plantations ? So large is the 

 capital now simk in tea, that no subject connected with 

 the business is too trivial to be discussed. The matter under 

 present consideration can be easily set at rest by chemical 

 analysis. Let, for instance, the constituents of some fresh 

 leaves from two or three indigenous and ordinary class of 

 hybrid bushes, each about two years old, be carefully investig- 

 ated, and let the result form the standard of excellence; it 

 will be easy then to deduce a formida by which any planter 

 may from time to time ascertain whether or not any deterior- 

 ation is perceptible in his plant. It is of little use seeking to 

 slove the problem by testing manufactured leaf, as even if 

 one tmiform system of manipulation were pm'sued in the tea 

 districts, atmosjiheric exigencies, begotten b> the different 

 sites on which the tea houses are built, would alone lead to 

 confusion. 



But, by whatever process, shoiUd a deterioration be 

 established beyond doubt, we should have then the question 

 to consider, first — whether such deterioration is due to the old 

 wood and knots formed by pruning, retarding the distribution 

 of the sap and consequent loss of the plant's vigor, or whether 

 the virgin constituents of the soil have been exhausted. In 

 either case science must be resorted to. AVere not our tea 

 planters hounded on to get as much as possible off the gardens, 

 there can be but little doubt that a periodical 'cutting back,' 

 almost to the ground, of certain portions of the concern, in 

 rotation, would be beneficial, inasmuch as it would result in 

 having fresh, straight, bearing stems, springing direct from 

 the roots with no laiots in them to retard tlie circulation of 

 the sap. AVe cannot help thinking however that an immense 

 amount of mischief is done by the severe 'cutting back 'of 

 young plantations. Ko doubt the first effect, where the soil 

 is rich and the gruwth of the plant vigorous, is a prolific yield, 

 but it is at the expense of future crops, and the formation of 

 knots and contortions which must retard the distiibutiou 

 of the sap. The subject of pruning has not received that 

 amount of attention, when considered from the .sap dis- 

 tribution point of view, that its importance demands. Our 

 jjlanters' efforts in the trimming line appear to have been 

 directed too much towards obtaining the largest possible 

 yielding area, without ensuriug that the distribution of sap 

 should be equal over that area. It has been in this way that, 

 in a bush say of five years old, while we may have a large 

 quantity of leaf, only a very small proportion is contributed 

 by fresh wood springiugdirect from the natural reservoir, the 

 root. Hence, we are of opinion that a considerable difference 

 in the strength of the usual properties that form the con-» 

 stituents of the flush will be found. Assuming our opinion 

 to be correct, it is manifest that in a garden of, say twenty 

 yt arc- old, the proportion of leaf taken from fresh beaiiug 



