264 THE EFFECT OF MOISTURE 



conditions being much reduced in size and rigidity.* 

 Lothelier t has made numerous observations in which 

 individuals of the same species were placed side by side, 

 some exposed freely to the air, and others kept moist 

 under a glass shade by a vessel of water. He found 

 that, for instance, Berberis vulgaris bore non-spinescent 

 leaves in a moist atmosphere, but spines and spines 

 alone in a perfectly dry one. Again, the shoots which 

 in Lycium barbarum, Ulex europceus, etc., would nor- 

 mally have formed thorns by arrest in development and 

 sclerosis, in a very damp atmosphere continued to grow, 

 and elongated into leafy branches. Microscopical ex- 

 amination showed that in the moist atmosphere the 

 parenchyma was only imperfectly differentiated into 

 spongiform and palisade tissue, whilst in dry air there 

 was a great arrest in the area of parenchymatous tissue, 

 but the palisade cells were well developed, and there 

 was a special consolidation of fibrous tissues. Again, 

 the common water-reed, Phragmites communis, when 

 growing in the unirrigated areas of the Nile Valley, 

 forms a stunted growth, with very short and sharp- 

 pointed leaves. " Close to the Nile, however, in Rhoda 

 Island, it grows nine or ten feet high, with long leaves 

 almost exactly like the plants in English rivers. 77 J 

 The effect of drought upon Dioscorea batatus (Yam) 

 has been carefully studied by Duchartre. Though 

 not allowed to have any water, some tubers of this plant 



*Rev. G. Henslow, " The Origin of Plant Structures," p. 40. 

 f See also " Recherches anatomiques sur les epines et les aiguillons 

 des plantes," Lille, 1893. 



} " Origin of Plant Structures," p. 41. 



Bull, de la Soc. Bot. de France, 1885, p. 156. 



