460 NOTES ON IIYDROCOTYLE AMERICANA. 



higher situated, to stretch themselves for reaching the ground, and I 

 believe that the most natural and for the plant the most useful develop- 

 ment of these branches from the axils of the leaves should be if they 

 all were developed as t uberi tenuis stolons. 



In regard to the internal structure of these organs, we see in the 

 stolons a very distinct cuticula, showing several frownings, an epi- 

 dermis of which the exterior walls of the cells are somewhat thick, 

 while the interior ones are thinner, and show a development almost as 

 collenchyma (Plate xlvii, Fig. 15). Stomata are present, but merely 

 in a small number. Inside the epidermis there is a bark composed of 

 parenchyma, with large cylindrical cells with very thin walls. There are 

 usually six vascular bundles, with a distinct duct, apparently merely 

 containing air, outside the phloeme, and there is no indication of any 

 mechanical tissue either forming a sheath or isolated groups. The cells 

 of the pith have the same shape and size as those of the bark. Starch 

 is present in the bark, but is only to be observed in the younger state 

 of the stoious; not at all by the older ones. 



The structure of the runners is almost the same as that of the sto- 

 lons, except that the stomata are more frequent; the cuticula does not 

 show frownings, and the cells of epidermis have thinner walls. The 

 bark and pith show the same structure, and the vascular bundles are 

 stronger developed and the groups of phloeme and xyleme largerj 

 The ducts outside the phloeme are also present in the runners. 



The stem shows the same general structure, but the cells of the 

 epidermis have attained a still greater thickness, and we find iuside 

 these a stratum of cells of an almost collenchymatous character (Plate 

 xlvii, Fig. 14). Otherwise the structure is the same as that of the 

 runners. 



The complete want of mechanical tissue in the stolons and ruuuers 

 seems to indicate that their persistence can not be very long, and their 

 anatomical structure shows a very uniform development in both. 



As to the structure of the tubers, we find that a transverse cut of ai 

 young tuber, formed this year, shows a large amount of starch, depos- 

 ited in the bark and pith, and in such a manner that its presence per- 

 fectly conceals the structure of the different strata of cells. But we may 

 see, however, very distinctly usually six concentric rather large ducts, 

 and these are of the same kind as those mentioned above. A trans 

 verse cut of an old tuber (Plate xlvii, Fig. 10), which has developed 

 a plant, and therefore has been deprived of its starch, shows the struct- 

 ure much better. The cells of the epidermis do not show any essential 

 difference from those of the runners, the structure of the bark and pit! 

 corresponds perfectly to the description given before, and the develop- 

 ment of the vascular bundles is very uniform. The ducts are to be seen 

 outside the phloeme, as in the stem, the runners, and the stolons; but 

 outside these we find an endodermis, forming a ring around the vascu 

 lar bundles, and whose cells show very thin walls, but somewhat iudis- 



