ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 269 



But cases of section at right angles occur but rarely in nature, a 

 great number of deviations being met with. One of the most frequent 

 causes of these deviations is the circumstance that growth in the tan- 

 gential direction, if it occurred by itself, would have a smaller intensity 

 than growth in the radial direction. The consequence is that the 

 originally orthogonal trajectors undergo a deviation towards the posi- 

 tion of most active growth ; this deviation increasing up to a maximum 

 point, and then again diminishing, and being of course accompanied 

 by a corresponding deviation from the rectangular section of the 

 layers. A second cause of the deviation, which, however, can 

 operate only in a cellular tissue, is the difference in pressure and 

 turgidity between different zones or surfaces, the trajectors then 

 deviating towards the side of smaller pressure, as is the case in 

 wounds. The trajectorial curves may be segments of circles, ellipses, 

 or parabolas. 



Schwendener's explanation of some points connected with these 

 phenomena differs from that of Sachs. The latter explains the direction 

 of the medullary rays, as well as that of the anticlinal rows of cells in 

 the apical region of stems and roots, on the same principle as that of 

 growth by cell-division, that the new cell-walls are applied to those 

 already in existence, thus causing rectangular section as a rule. 

 Schwendener, on the other hand, considers cell-division as a special 

 process of its own, while the formation of trajectorial rows, even in 

 cellular tissue, is governed by the same mechanical principles that 

 regulate the direction of the rows of molecules in starch-grains, thick- 

 ened cell-walls, &c. The parallel course of the trajectors and of the 

 cell-walls increases the effect, but is not the original cause. 



Morphology and Physiology of the Leaf.* — K. Goebel treats in 

 detail of the morphology of the leaf, especially, in the first place, in 

 reference to the " eataphyllary " leaves {Niederhldtter of Braun), 

 which he regards as the result of true metamorphosis by degradation 

 of the foliar leaves. Two forms of these leaves are especially 

 discussed, the leaf bud-scales and the rhizome-scales. The former 

 are especially characteristic of the deciduous woody plants of cold 

 climates (occurring also in some evergreen Coniferas), though they are 

 not universally present ; when absent, their function of protecting the 

 growing bud against wet, drought, or cold is performed by a felt of hair, 

 or some other special contrivance. The hairs in this case are almost 

 invariably peltate. The transition from these scale-leaves to 

 ordinary foliage-leaves is always sudden. In some cases the function 

 of the bud-scales is performed by the stipules of the earliest leaves. 

 Cataphyllary leaves occur in some Vascular Cryptogams, as in some 

 species of Isoetes, Stridhiopteris, and Osmunda, according to Prantl. 



Secondly, the author treats of certain cases of habitual anisophylly, 

 such as occurs in Selaginella, most foliose Jungermanniacefe, and many 

 flowering plants. This phenomenon is traced to special relationships 

 of the plants in question towards light. 



* Bot. Ztg., xxxviii. (18S0) pp. 753-60, 769-78, 785-95, 801-15, 817-26. 

 83.5-45 (1 pL). 



Ser. 2.— Vol. I. U 



