IN THE MOTOR OKGANS OP LEAVES. 



39 



insufficient root-supply of water; but, of course, excessive transpiratory loss will produce 

 similar effects, as the following experiment shows: 



Experiment IV. — Two leaves of Kalanchoe laciniata, each weighing exactly 1038 

 grammes, were taken. One of them (a) was placed in a simple sealed chamber, and 

 the other (b) in one including capsules containing sulphuric acid and chloride of 

 calcium. The chambers were placed side by side in front of a window, so as to secure 

 their equal exposure to light. Twenty-four hours later, (a) remained quite plump and 

 rigid and weighed 10*26 grammes; whilst (b) was flaccid throughout, slightly browned at 



some points on the margin, and weighed only 8*35 grammes. Both leaves were now 

 removed and planted with the bases of their petioles in moist earth, and hero (b) shortly 

 regained its turgidity save in the browned areas. Here the influence of root-supply was 

 eliminated, as neither leaf had any source of supply whatever, and the excels of loss in 

 weight shown by (b) as compared with (a) is thus to be credited solely to the action of 

 excessive transpiratory loss. The loss in weight in (a) amounted to 0*12 and may be 

 taken to indicate the quantity of water required to saturate the air of the chamber 



e> 



with any expended in the course of assimilatory processes. This 



of 1-91 in (£), ascribable to excess of transpiratory loss determined by the aridity of the 



of the chamber. 

 It is the want of equilibrium between supply and 



essential factor 



determining the result; not any absolute amount of either. Tho root-supply during the 

 course of the night must, as the soil cools, tend to diminish ; and yet it secures the renewed 

 turgidity of tissues which have wilted during the day, because it no longer has to 

 contend with transpiratory loss. Once the dew-point has been arrived at, transpiratory 

 loss comes to an end ; and hence the continued supply, although not so great as during 

 the day, is able to restore turgescence in spite of the diminished manufacture of osmotic 

 products incident on absence of solar stimulation of the protoplasm. That it is the 

 cessation of transpiration, and not any other nocturnal condition, which is the efficient 

 cause, is indicated by the fact that, on cloudy and dewless nights, recovery does not 

 occur, but that, apart from increased water-supply, the wilting is carried on continually 

 into the next da v. The same thing is, as we shall see when we come to consider 



yctitropic phenomena, very clearly indicated also by the fact that the nocturnal con- 



dition in tissues exhibiting these attains its maximum in the early part of the night 



and then goes on gradually reverting, up to a certain point, towards the d 



In endeavouring to account for any particular fluctuations in turgescence and for the 

 effects following these, we have to consider not merely the conditions of protoplasmic 

 activity regulating the manufacture of osmotic products, but also extrinsic conditions 

 affecting general supply and loss of fluid. The fullest stimulation of the protoplasm may be 

 present (as in hot dry afternoons) coincidently with very imperfect turgescence, owing 

 to deficient supply and excessive transpiratory loss of fluid; on the other hand, a high 

 degree of turgescence may accompany defective stimulation, as it does during the latter 

 part of dewy nights, because of the coincident continued supply and abolished loss of 

 water. The maximum of turgescence in tissues in which turgidity is dependent on con- 



_ued vitality is only reached where they are exposed to coincident maximum stimulat 

 of" their protoplasts, to abundant supply and to abolished loss of water; but all minor 

 degrees may be present where these various factors are otherwise combined. In any 

 ordinary plant under normal circumstances ceaseless insensible fluctuations in turgescence 



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