826 SUMMARY OF CURRENT RESEARCHES RELATING* TO 



Causes of the Fall of the Leaf.* — Dr. H. Molisch gives the follow- 

 ing as tho more important results of observations on this head : — 



If transpiration is suddenly stopped in branches which ordinarily 

 transpiro strongly, the leaves fall ; whilo plants which thrivo in a 

 moist atmosphere often preserve their leaves for a long time in 

 saturated air. A not too rapid but continuous diminution of tho water 

 in tho soil tends to the formation of the separating layer, and in many 

 cases to tho fall of the leaf, which is then greatly favoured by tho 

 sudden saturation of tho soil. Tho fall takes place indifferently 

 whether tho withering is caused by increased transpiration, by in- 

 sufficient supply of water, or by both causes ; but if the withering 

 takes place too rapidly, the leaves dry up before the separating layer 

 is formed. Cut branches which transpire slowly shed their leaves 

 even when lying on tho ground. An insufficient supply of water is 

 also the causo of cut branches dipped in water losing their leaves 

 earlier than when growing on tho tree, and also of plants shedding 

 their leaves when moved from the open soil into pots. Stagnant 

 water in the soil injures tho roots and causes plants partially or 

 entirely to loso their leaves. The same result ensues from want of 

 light. This is shown most by strongly transpiring plants with 

 herbaceous leaves, such as Coleus, less by those with coriaceous 

 strongly cuticularized leaves, such as Azalea, Rhododendron, and 

 Abies pectinate, scarcely at all by evergreen conifers or by the box. 



The influence of temperature on the fall of leaves is very com- 

 plicated. It acts both directly, and indirectly by influencing tho 

 transpiration. Oxygen is an essential condition of the fall of the leaf; 

 leaves immersed in water become detached much later than those 

 growing in moist air. 



The gum-ferment discovered by Wiesner f occurs in many plants 

 in large quantities in the separating layer ; this renders it probable 

 that the absorption of the middle lamella, and the isolation of the 

 cells, is completed by a cellulose-transforming ferment, assisted by 

 organic acids. 



Transpiration of Plants.^ — Sig. E. Pavani gives a full account 

 of previous researches, as well as of fresh observations of his own, on 

 the absorption of water by plants, especially trees, the amount of 

 transpiration, and the influence on climate. This latter he considers 

 to be greater than is generally supposed, and chiefly from the absorp- 

 tion of heat which must necessarily take place in the conversion of 

 the water into vapour. Conifers, from the small amount of surface of 

 their leaves, exercise much less influence, and are much more adapted 

 to dry climates and an arid poor soil, than trees with larger leaves. 



Causes of Torsion.§ — Herr F. Kreuter details the reasons which 

 have led him to the conclusion that the torsion of stems is due to the 



* SB. K. Akad. Wiss. Wien, Feb. 11, 1886. See Bot. Ceutralbl., xxv. 

 (1S86) p. 393. 



t See thia Journal, ante, p. 106. 



% Boll. Soc. Adriat. Sci. Nat. Trieste, ix. (1886) pp. 17-43. 



§ Naturforscher, xix. (18S6) pp. 211-2, 222-3, 231-2. 



