ZOOLOGY AND BOTANT, MICKOSCOPY, ETC. 763 



layers of this cylinder are converted into root-cap, the lateral external 

 layers into dermatogen and root-cap. 



Permeability of the Epidermis of Leaves for Gases.* — M. L. 

 Mangin gives the details of a number of experiments made to determine 

 the permeability of the epidermis for gases. The following are his 

 conclusions : — 



(1) That the permeability of the epidermis of aerial leaves is very 

 limited ; ordinarily feeble for plants with persistent leaves, it is rather 

 more considerable in plants with deciduous leaves. (2) In leaves in 

 which the upper and lower surfaces are dissimilar, the permeability of 

 the lower epidermis is greater than that of the upper. (3) The 

 permeability of the epidermis of submerged leaves which are destitute of 

 stomata is very great, — five, ten, or even twenty times more than that of 

 aerial leaves. (4) The permeability of cutinized surfaces is notably 

 weakened by the waxy matter which is found in the cuticle of all leaves ; 

 this applies to submerged as well as to aerial leaves. 



Influence of the Turgidity of the Epidermal CeUs on the Stomata.t 

 — Dr. E. Schaefer contests the theory of Schwendener | that the chief 

 cause of the widening and narrowing of the cleft of the stomata is the 

 changing pressure exercised on them by the varying turgidity of the 

 epidermal cells which adjoin the guard-cells. From observations on 

 a number of plants {Polygonum, Lilium, Potamogeton, Azolla, &c.), he 

 comes to the conclusion that the stomatic apparatus is endowed with an 

 independent function, and that this function is rendered possible only by 

 the changes in the turgidity of the guard-cells. It must, however, be 

 admitted that the turgidity of the neighbouring cells of the epidermis 

 prevents the free expansion of the guard-cells. The width of the cleft at 

 any particular time is therefore the resultant of two opposing forces, the 

 stronger of these being the turgidity of the guard-cells, the weaker that 

 of the adjoining epidermal cells. The observations on the stomata of 

 Azolla were especially instructive, as here the opening and closing of the 

 cleft takes place in the ordinary way, and must be brought about by 

 internal forces only, as the thickening-bands which occur in other plants 

 in the neighbouring epidermal cells are here wanting. In grasses, also, 

 the case is very similar, the changes in the width of the cleft being 

 obviously due to forces which have their origin in the guard-cells. 



Anatomy of Spines.§ — Under the term spine Herr E. Mittmann in- 

 cludes all structures which end in a sharp point, and which are adapted 

 by their anatomical construction for the protection of the plant, and for 

 the dissemination of the seeds or fruits through the agency of animals. 

 The following are, with some exceptions, the general anatomical charac- 

 teristics of all spines : — A strong development of the mechanical tissue ; 

 its situation near the surface, and increase in strength from the base 

 towards the apex ; the strong thickening and lignification of the walls of 

 the cells of which this tissue is composed. A corresponding reduction 

 of the assimilating and conducting tissues. The peculiarity, especially 

 striking in stem-spines, that growth continues longest at the base of the 



* Comptes Eendus, cvi. (1888) pp. 771-4. 



t Pringsheim's Jahrb. f. Wiss. Bot., xis. (1888) pp. 178-205 (1 fio-.). 

 X See this Journal, 1882, p. 216. 



§ ' Beitr. z. Kenntniss d. Anat. d. Pllanzenstachelu,' 43 pp., Berlin, 1888. See 

 Bot. Centralbl., sxxiv. (1888) p. 359. 



