CHAPTER V. 



EXCESSIVE HUMIDITY. 



The Mode of Growth With Continued Atmospheric Humidity. 



Older works have called attention to the fact that the structure and 

 functions of individuals are altered by the influence of a high degree of 

 atmospheric humidity in the same way as by the removal of light. Accord- 

 ing to experiments of Vesque and Viet^, plants grown in moist air have 

 longer, less branched roots, more delicate stems, leaves with longer petioles 

 and smaller blades. The walls of the epidermal cells are less wavy; the 

 cell rows of the mesophyll somewhat less numerous and without differ- 

 entiation into palisade parenchyma. The whole tissue of the leaf grown 

 in moist air is in every respect more uniform, while in dry air the dift'erence 

 between palisade and spongy parenchyma is clearly seen. The vascular 

 bundles in the internodes are much more developed in dry air. This refers 

 not only to the diameter of the whole bundle, to the number of ducts and 

 their diameter, but especially to the hard bast fibres which may occur 

 abundantly in dry air and be entirely lacking in moist air. Duval-Jouve- 

 observed with grasses that dry, hot situations increased the development of 

 the bast bundles, while, in moist places, this development is retarded. The 

 authors named quote Rauwenhoff^, who describes etiolated plants in this 

 way. In comparative experiments with dry and moist air, under light bell- 

 glasses, as well as shaded ones, it was found that, in darkness but in dry 

 air, the plants were less spindling than those grown in the light in moist air, 

 from which it is concluded that the form of the etiolated plants is due chiefly 

 to deficient transpiration. 



Brenner^ expresses the same theor}^ In his experiments with Cras- 

 sulae, he observed a tendency to decrease the succulency of the leaves in 

 moist air, but to increase the upper surface. The cells of the stems were 

 actually elongated. Wiesner^ also found that the leaves of Sempervivum 



1 Vesque et Viet, Influence du Milieu sur les vegetaux. Annales des scienc. 

 nat. Sixieme serie. Botanique t, XII, 1881, p. 167. 



2 Botan. Jahresbericht 1875, p. 432. 



3 Annal. d. scienc. nat. 6 ser. V, p. 267. 



4 Brenner, W., Untersuchungen an einigen Fettpflanzen. Just's Bot. Jahresb. 

 1900, p. 306. 



5 Wiesner, Jul., Formveranderungen von Pflanzen bei Kultur in absolut 

 feuchten Raumen. Ber. d. Deutsch. Bot. Ges. 1891, p. 46. 



