988 SUMMABY OF CURRENT RESEARCHES RELATING TO 



to that of a very young root. If a transverse section be made in the middle 

 of the swelling, it will be seen that the tissues of the root are modified. 

 Some cells of the pericycle elongate radially, and divide by radial and tan- 

 gential septa ; the cells composing the endoderm and internal portion of the 

 cortex also elongate and divide. In the middle portion of the cortex a 

 separation is also produced. Thus it will be seen that, as in the case of 

 Melampyrum,* the pericycle, endoderm, and cortex take part at the same time 

 in the new formation. Erom this point the growth of the sucker is rapid. 

 The author concludes by stating that the development of a sucker differs 

 from that of a root, and its structure only accords with that of a root in a 

 few characters. 



Golour of Coloured Leaves.f — Prof. T. W. Engelmann has investigated 

 the cause of the colouring of the leaves in a large number of plants in which 

 they are normally coloured, and its relationship to the decomposition of 

 carbon dioxide in the light. 



The colouring may depend on two different causes : on a variation in 

 the colour of the assimilating chromophyll-bodies, or on the occurrence in 

 the leaf of special pigments in addition to the normal chromoplasts. In the 

 first case, the colouring appears to be invariably light, and either pure 

 yellow or yellow-green, with easy transition to ordinary chlorophyll-green ; 

 in the second case it is usually red-brown, dark purple-brown, purple-red, 

 or violet. In the first group of cases there appears to be frequently a 

 definite quantitative relationship between the amount of colouring matter 

 and that of chlorophyll. The colouring matter of the leaves of the yellow 

 variety of Sambucus nigra was especially investigated and described. The 

 yellow tint does not appear to be due here to a pure xanthophyll, but to a 

 mixture containing a small quantity of true chlorophyll and of chlorophyllan. 

 In more refrangible light (about A = • 53 /x), the yellow cells decompose 

 relatively, if not absolutely, more carbon dioxide than the green, while in 

 red and green light the green cells decompose, both absolutely and relatively, 

 more than the yellow. 



In the second group the seat of the pigment is usually the cell-sap ; 

 less often the cell-wall. In the latter case the colouring is mostly confined 

 to small portions of the surface, causing variegated leaves. Of leaves 

 coloured by a soluble pigment, about fifty kinds were examined. These 

 may be divided into two groups, connected with one another by intermediate 

 forms : those in which the leaves are normally coloured during the whole 

 or the greater part of their existence, and those which are coloured only 

 when young. The colouring is, in both these cases, usually, but not always, 

 spread over the whole surface of the leaf. 



With regard to the distribution of the pigment in the component tissues 

 of the leaf, all the cells of the epidermis and its appendages, as well as the 

 assimilating parenchyma, may contain coloured sap. In other cases only 

 some of the epidermal cells of definite position are coloured. A red pigment 

 is commonly contained exclusively in the assimilating tissue, especially in 

 the palisade cells. That cells containing a purple sap can decompose 

 carbon dioxide as energetically as those which contain pure chlorophyll is 

 shown, among other examples, by the great size attained by the copper-beech, 

 and the vigour of growth of the various species of Coleus. Neither the size, 

 form, arrangement, colour, nor number of the chlorophyll-grains presents 

 any peculiarity in such leaves. Only those rays appear to be absorbed by 



* Cf. this Journal, ante, p. 778. 



t Bot. Ztg., xlv. (1887) pp. 393-8, 409-19, 425-36, 441-50, 457-70 (2 pis.), and 

 Arch. Ne'erland. Sci. Exact, et Nat., xxii. (1887) pp. 1-57 (2 pis.). 



