50 PROCEEDINGS OF THE AMERICAN ACADEMY 



the host, rising at places in external prominences for the purpose of 

 producing fruit. In the external portion of the frond, the proportion 

 of" the filamentous cells to the globose ones is very variable. Some- 

 times the former encroach far on the latter, while again the globose 

 cells may entirely exclude the filamentous ones from the external 

 frond. In the same way, the size of both kinds of cells varies a great 

 (leal ; in fact, in all the structures of the frond, even in rare cases in 

 the fruit, a great diversity in appearance may be noticed. 



Besides the kinds of cells already described, the peripheral cells 

 of the frond present a very characteristic appearance. Nearer the 

 outside the cells are seen to be smaller, more nearly spherical in 

 shape, and more regularly arranged, than in the rest of the frond. 

 The gelatinous intercellular substance is also considerably diminished 

 in quantity in this region. Those cells which form the extreme 

 outer layer are still different in shape from any of the others. They 

 are somewhat elongated and pyriform, the smaller ends being directed 

 inwards (Figs. 5, 8). They constitute the growing part of the frond, 

 as will be described later, in discussing the development of the plant. 

 The contents of these outer cells is more granular than that of the 

 others, and the nuclei are more distinct ; in fact, they present all the 

 appearances characteristic of growing cells. Directly outside of 

 the pyriform cells there is a thick sheath of cellulose, which covers 

 and protects the whole frond (Figs. 8, 17). This outer skin of cel- 

 lulose is not formed by the fusion of the exterior walls of the pe- 

 ripheral cells ; they are only loosely connected with it, and may be 

 detached from it without injury. The cellulose sheath may be dis- 

 sected off in large pieces, when it is seen to be almost structureless, 

 except for the depressions left by the cells which had formerly been 

 attached to it, and for the irregular blotches of brownish red color- 

 ing matter in it. By this means the pigment to which the color of 

 C. PoJijsiphonice is due may be seen to be contained almost entirely 

 in this external covering. Sometimes the peripheral cells may be 

 also tinged with brown, while in one or two cases the whole tissue 

 of the frond partook of this color. The sheath is nothing more 

 than a thickening of the gelatinous intercellular substance on the out- 

 side of the frond. This gelatinous substance is itself but a modifica- 

 tion of a portion of the walls of the cells, and gives a cellulose test 

 with chloriodide of zinc as well as the sheath. As in the case of the 

 other cells of the frond, those near the periphery are subject to some 

 variation. In those fronds where the filamentous cells extend into 

 the external protuberance of the frond, the peripheral cells partake 



