272 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and pliyllodes tlie former are equally strongly developed on both sides 

 of the leaf. The former therefore occupy, the portion of the cell that 

 is most, the latter that which is least exposed to the action of light ; 

 and this is connected with a difference in the distribution of the 

 chlorophyll-grains in the two kinds of cell. 



In the cells of the spongy parenchyma the chlorophyll- grains may 

 occupy two positions. With a small amount of light they cover the 

 cell-walls parallel to the surface of the leaf, or occupy a superficial 

 position; more intense light causes them to assume the pro^Ze position 

 on the walls at right angles to the surface. In the palisade-cells, on 

 the contrary, whatever the intensity of the light, the chlorophyll- 

 grains can occupy only one position, the profile. 



The chlorophyll-grains in the palisade-cells receive the rays of 

 light direct ; those in the spongy parenchyma, on the other hand, 

 receive those only which have been weakened by passing through the 

 upper layers. This disadvantage is, however, to a certain extent 

 compensated by the superficial position of the grains in the latter. 

 The palisade-cells, with their cell-cavity elongated in a direction 

 vertical to the surface, allow a considerable amount of light to pass 

 through. 



The thin leaves of plants which grow in deep shade, like Oxalis 

 acetosella, Mercurialis perennis, and JDentaria hulbifera, have scarcely 

 any palisade-tissue, while on the other hand, in the thick, stiff leaves 

 of plants which are freely exposed to the sun, the spongy parenchyma 

 is reduced to a minimum. Even the same species shows a difference 

 in this respect according to the position in which it grows. 



Heliotropism of the Ivy.* — The peculiar relations of ivy-cuttings 

 to the light are well known, according to which they are negatively 

 heliotropic when more strongly, positively heliotropic when less 

 strongly illuminated. To determine the causes of its heliotropic 

 phenomena. Dr. C. Kraus has experimented on three cultivated 

 varieties. The ivy is peculiarly a shade-plant, and its growing 

 shoots are therefore unusually sensitive to light. The difference 

 between the upper and the under side of ivy shoots appears to be 

 simply an excessive manifestation of the contrast so often manifested 

 between the illuminated and the shaded side of branches ; while the 

 contrast, in those shoots where the phyllotaxis is biserial, between 

 the sides which bear the leaves and those destitute of them, appears 

 to be of a much more deep-seated and organic nature. The primary 

 cause of the peculiar heliotropic behaviour of the ivy is stated by 

 the author to be its specially great sensitiveness to light, in conse- 

 quence of which internal changes take place in its shoots under strong 

 light, the object of which is to obtain situations where the intensity 

 of light is most favourable. The same conditions which relax and 

 dwarf the leaves of the ivy in strong light, tend also to cause the 

 stems to seek the most favourable intensity of light. 



Pinguicula alpina an Insectivorous Plant.f — Dr. J. Klein has 

 made an exhaustive examination of the anatomy and physiological 



* ' Flora,' Ixiii. (1880) pp. 483-9, 499-514, 525-8 (1 pi.). 

 t Cohn's Beitr. Biol. Pflanzen, iii. (1880) pp. 163-85. 



