592 SUMMARY OF CURRENT RESEARCHES RELATING TO 



plant must pass the protoplasm of the endodermis, for it cannot permeate 

 the walls. 



The endodermis thus functions as a living membrane, completely 

 surrounding the central cylinder of the root. This membrane, the 

 present experiments prove, exercises a distinct selective power, which is 

 not merely quantitative, dependent upon the strength of the solutions 

 concerned, but qualitative also. The author points out, however, that 

 this latter selective power appears to be of little use to a plant growing 

 in normal conditions — for the soil does not contain substances incapable 

 of penetrating the protoplasm. 



Youth-form of Leaves and its Significance.* - Th. Nicoloff 

 discusses the phyletic significance of the form adopted by young leaves 

 of certain plants, either in seedlings (Pothos celatocaulis, Acer Negundo, 

 Fraxinus excelsior, etc.), or on mature branches, normal or adventitious 

 (Jwjlans). He concludes that this youth-form does not invariably 

 afford a criterion of affinity. In some cases it is due merely to special 

 conditions of environment (Pothos), and change in leaf -shape on normal 

 branches may be induced by change in the environment, as shown by 

 certain experiments of Goebel ; in other cases, such as certain aquatic 

 monocotyledons, the phenomenon is not so readily explained. But 

 there is no doubt that in some instances some connexion exists between 

 the youth-form of leaves and the normal leaf -form of allied species, 

 particularly in the case of leaves which arise upon adventitious branches. 

 The author suggests that during the formation of a species, the new 

 specific characters, which are the expression of a changing environment, 

 appear first in the later stages of ontogeny, gradually working back, in 

 the process of time, to the earlier stages. The earliest stages of 

 ontogeny may thus tend, in certain cases, to lack new characters, and so 

 display the ancestral characters. In some cases, even, where evolutionary 

 progression has not advanced too far, the ancestral form may be induced 

 by a return to the old conditions — as Goebel's experiments show — but 

 this is not often the case. Adventitious buds are endogenous, arising 

 from the pericycle ; this tissue, being deep lying, is not effectively 

 exposed to the new environmental conditions, and so retains for a 

 relatively long period the disposition to " revert " to ancestral characters. 



The " youth-form " displayed by leaves on adventitious branches, 

 then, affords the safest guide to ancestry. 



Osmotic Pressure in Plants. f — H. H. Dixon and W. R. G. Atkins de- 

 scribe a new method of determining the freezing-point of small quantities 

 of liquid with , accuracy by a thermo-electric process ; this they apply 

 to the sap of various plants, and thence calculate the osmotic pressure. 

 The relation between the latter (P) and the consequent depression 

 (A) in the freezing-point of the sap has been given by Nernst as 

 A x 12*08 = P, in atmospheres. The authors conclude that osmotic 

 pressures are variable with the species and individual alike, and the 

 variation in the same individual under varying circumstances may be 

 very considerable — e.g. 24 "58 atm. to 11*5S atm. in Lilac. Leaves of 

 the same individual under similar conditions have the same osmotic 



* Rev. Gen. Bot., xxii. (1910) pp. 113-24 (figs.). 



t Proc. Roy. Soc. Dublin, xii. (1910) pp. 275-311 (fig.). 



