92 SUMMARY OF CURRENT RESEARCHES RELATING TO 



wliile in the former the walls become straigliter aud thicker, the 

 latter being especially the case with the cuticle. The author believes 

 the production of stomata in these cases to be the direct result of 

 the accumulation of nutrient substances. 



Influence of Light on the Assimilating Tissue of Leaves* — As 



a sequel to the investigations of Haberlandt f and others on the struc- 

 ture of the palisade-parenchyma or assimilating tissue of leaves, 

 H. Pick has studied the direct influence exercised on its development 

 by the intensity of the light. In opposition to the statement of Stahl 

 that the palisade-parenchyma is wanting on the upper surface of 

 leaves growing in the shade, the author finds that it is in most cases 

 present (in the case of beech-leaves and Hieracium villosuni), but that 

 in proportion to the want of light the length of the cells (in the 

 direction at right angles to the surface) diminishes. In other cases 

 the cells of this layer are round or even elongated in a direction 

 parallel to the surface. Direct influence of sunlight on the development 

 of the palisade-])arenchyma was only rarely observed ; in one case 

 (Osmunda regalis) different leaves of the same plant, and even different 

 parts of the same leaf, exhibited differences in this respect. 



The author regards, however, the palisade form of cells as, in most 

 cases, an inherited peculiarity of the mesophyll of the upper side of 

 the leaves exposed to the action of the sun, and very rarely as affected 

 directly by the action of light ; the peculiar form of cell is partially 

 developed even in the bud-condition. This conclusion is perfectly in 

 accordance with the fact that the palisade-tissue is present also, 

 though less strongly developed, on the under side of the leaf. The 

 formation of this assimilating tissue is not prevented by a dense 

 covering of hair, the object of which is not so much protection from 

 too intense light as the prevention of too rapid transpiration. 



In the case of " compass-jjlants,"' the structure of the leaves of 

 which has recently been described by Stahl, ;{: light appears to exercise 

 a direct influence on the form of the assimilating tissue in the nearly 

 erect leaves, the palisade-cells being sometimes formed on one, some- 

 times on the other side, according to the relative intensity of the 

 light. 



The assimilating organs have the power of always assuming a 

 definite position with respect to the direction of the incident light, 

 either by the heliotropism of the leaf or leaf-stalk, or by the cells 

 themselves placing themselves in a position at right angles to the 

 incident rays, this latter taking i)lace with stalked leaves which are 

 less able to alter their position in relation to light. 



Development and Structure of Sieve-tubes.§ — In pursuance of 

 his previous investigations of this subject,] | E. Eussow has now 



* Bot. Centralbl., xi. (1882) pp. 400-6, 438-46 (1 pi.), 

 t See this Journal, ii. (1882) p. 368. J Ibid., p. 373. 



§ SB. Dorpater Naturf. Gcsc'lls., 1882, pp. 257-327. See Bot. Ccutralbl., 

 xi. (1882) p. 419. 



II See this Journal, ii. (1882) p. 218. 



