78 T HE CAUSES OF FLUCTUATIONS IN TURGESCENCE 



and the earth gently washed away under a stream of running water, so as to free the 

 roots without injury. The entire plant was next weighed, and the weight was ascertained 

 to he only 2'03 grammes. A certain amount of the increment in weight which was 



sustained by the acid duriDg the course of the experiment must, of coursa, be credited 

 to the atmospheric humidity of the air of the chamber; but as the latter was a very 

 small one, the mass of water furnished by transpiratory loss during twenty-four hours 

 must have weighed nearly twice ns much as tiie entire plant furnishing it. 



Experiment II— A plant with fire leaves and weighing 2*32 grammes was treated 

 in exactly the same fashion as that in the previous experiment. At the close of four 

 hours' exposure the sulphuric acid indicated a gain of Vbc.c. of water, equivalent to a 

 mass of more than half the weight of the entire plant. (Vide Appendix 13.) 



The extreme activity of transpiratory loss is further very clearly indicated by the 

 phenomena which attend exposure to direct sunlight. When the soil and air are dry 

 continued exposure to the rays of the sun is followed by a complete assumption of the 

 nocturnal position by the various parts of the leaves, and, even when soil and air are 

 both comparatively humid, a very brief exposure to direct insolation is sufficient to give 

 rise to various degrees of elevation of the pinnules. The phenomenon is particularly 

 striking when it occurs in a large bed of the plant, a portion of which is artificially 

 shaded, owing to the contrast presented by the fully expanded leaves within the 

 limits of the shaded area and those in the insolated one. Taking the experimenta 

 and natural data together, they appear very clearly to indicate that the diurnal 

 position of the leaves is one of very unstable equilibrium dependent on a definite relation 

 between supply and loss of waiter, and liable to be disturbed by any conditions implying 

 either diminution in supply or increase in loss. The phenomenon of partial assumption 

 of the nocturnal position by the pinnules under the influence of direct insolation, even 

 where the humidity of the soil is very high, may be taken as indicating the existence 

 of a definite limit to the activity of root absorption ; but from experimental data, which 

 will be furnished presently, it appears not unlikely that another factor comes into play 

 to produce the result. The elevation of temperature which attends direct insolation not 



gives rise to increased transpiratory loss, but must also cause an expansion of the 

 gaseous contents of the water- conducting system which may well act as an obstruction 

 to the passage of liquid along it; so that the departure from the diurnal position may be 



r, not merely to increased transpiratory loss, but to diminished supply of water. 

 That this is actually the case is certainly suggested by the fact that direct insolation, 

 even within a closed moist chamber, is attended by a certain amount of elevation of the 

 pinnules. Here, of course, the atmosphere is excessively humid, and transpiratory loss is 

 correspondingly reduced ; but assimilatory decomposition of water continues, and if this 

 coincides with obstruction to the passage of liquid along the water- vascular system, a 

 certain amount of decrease in turgescence must necessarily ensue. The decrease, how- 

 ever, is inconsiderable, and under normal circumstances, neither alone nor in co-oporation 

 with decrease due to coincidently increased transpiratory loss, docs it suffice to determine 

 the full assumption of the nocturnal position. Jt is only unler the coincident influences 

 of dry soil, dry air, and insolation that the full nocturnal position is spontaneously 

 attained during the day: in other words, it is only attained under circumstances which 

 imply defective root-supply, gaseous obstruction of the water- vascular system, and 

 excessive transpiratory loss. 





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