270 Transactions of the 



stratum of cells, the framework of which is constituted by network 

 of extremely slender coloured or chlorophyll cells, into each of the 

 meshes of which we might fancy one of the vesicular cells had been 

 dropped. By section we see that the relative position of these two 

 kinds of cells to each other may vary, for the chlorophyll cells may 

 lie midway between the anterior and posterior surface of the leaf, 

 and their section shows us that they are lenticularly compressed, or 

 they may take part in forming the anterior or posterior surface of 

 the leaf, their transverse section being triangular, so that they re- 

 semble a wedge pushed in between each pair of hyaline cells : minute 

 as this structure is, we must admit its importance, since it originates 

 in the fundamental formation of the leaf. 



The hyaline cells are more or less united by their adjacent walls, 

 and nearly always contain threads attached to their internal walls ; 

 these threads may form complete spirals, composed of one or several 

 fibrils, or they may be broken up into rings and spiral fragments, 

 and sometimes run across diagonally so as to unite two spirals. 

 Threads, however, are not always present in all the leaves, for in 

 S. Jimhriatum they are wanting in both the stem and peduncular 

 leaves, and others have them in one part of the leaf while they are 

 absent from the rest ; in 8. (Isocladus) macropliyllum no threads 

 are seen except those forming a ring round the orifices of the pores. 

 The threads are firm and intimately united to the inner wall of the 

 cells, so that in 8. suhsecundum, the walls of the hyaline cells are 

 strongly contracted by them. 



The apertures or pores are most abundant on the back of the 

 leaf, and stand near the adjoining cell- walls ; they vary in size and 

 number according to the species, and no doubt originate by the 

 resorption of the delicate cell-wall, within the boundary of a small 

 thread-ring. Besides these, Eussow calls attention to larger open- 

 ings which become visible after treatment with iodine, and indicating 

 more extensive resorption of the cell-membrane. Thus in the lower 

 part of a branch leaf of 8. jimhriatum so treated, these large 

 apertures reach across the whole width of the cell, and stand be- 

 tween each pair of thread sj)irals ; in the corresponding leaves of 

 the nearly allied ;S^. Girgensohnii this resorption appearance does 

 not occur. In leaves from the pendent branches of 8. intermedium, 

 a hole is always seen at the apical end of each cell. In 8. Lind- 

 hergii, jimhriatum, and Girgensohnii, whose stem leaves are fringed 

 at the apex, this appearance is due to complete resorption of the 

 membrane of the hyaline cells, and consequent projection of the 

 intermediate parenchym cells. 



The chlorophyll cells of peduncular leaves usually have de- 

 ficiencies in the thickening layers of their walls, and these standing 

 opposite to each other, resemble imperforate dots, not unlike the 

 dotted pleurenchyma of coniferous wood; a similar condition is 



