80 SCIENCE PROGRESS. 



culture experiments (Duchartre and Henslow, loc. cit., p. 57) 

 show that sclerenchyma may be directly diminished by a 

 more moist atmosphere. 



The occurrence of gum is not so clearly dependent on 

 the climatic conditions ; its use in these forms is, however, 

 obvious enough, for all apertures by which water might be 

 lost are, so to speak, gummed up. This is quite similar in 

 physiological action to the drops of mucilage or gum which 

 hermetically seal the vessels exposed by cutting across a 

 branch of any ordinary deciduous tree. 



It is true that the production of gum is known to be 

 most abundant in a dry and hot season, but according to 

 the explanation given by Tschirch, loc. cit., p. 2 1 1 (and an 

 identical account has been given me by Mr. Malcolm Dunn 

 as the result of experience), this is due to the gum being 

 squeezed out by the contraction of the bark following on a 

 wet period, during which the masses of gum in the bark 

 are greatly swollen. I cannot find any explanation of the 

 actual cause of the change of cellulose into gum, but Mr. 

 Malcolm Dunn states the general opinion that it is abundant 

 after a severe shaking of the trees, as, for example, in a violent 

 wind. Such places as those here treated of are certainly 

 exposed to wind (otherwise they would be covered by 

 forest, according, that is, to my experience), and it is possible 

 that the wind may have assisted in starting gum formation ; 

 but if, as is not unlikely, the wind acts indirectly by straining 

 the layers of the cell walls, it seems more probable that the 

 fierce heat of the fire, causing sudden and violent shrinking 

 and warping of the bark, strains the cell walls in the same 

 manner. This may of course be quite unproved, but the 

 facts are sufficiently interesting to justify further research. 



G. F. Scott Elliot. 



