414 PRlSfARV ARRANGEME.yT OF TISSUES. 



Trapa natans has no cap, although, according to Reinke, it shows at least an in- 

 dication of one in the form of isolated tangential divisions of the epidermal cells. 



Sect. 123. As often had to be mentioned in former sections, the limiting 

 layers, where parenchymatous masses adjoin non-equivalent groups of tissue, are 

 often developed in the form of distinct layers, which stand to the latter in the 

 relation of sheaths. The hypodermal layers of foliage-leaves already described, as 

 well as the limiting layers between the sharply-bounded central portion and the chlo- 

 rophyll-parenchyma of centrically constructed leaves, belong essentially to this cate- 

 gory (comp. Fig. 185, p. 381). But the endodermis of many aerial roots, already 

 considered in Sects. 27 and 56, and the parenchymatous sheaths, which stand in 

 immediate relation to the vascular bundles or to definite groups of them, still require 

 to be especially mentioned here. According as the one or the other of these latter 

 relations exists, and thus, according to their position, we have first to distinguish 

 those parenchymatous masses, each of which surrounds one vascular bundle,— and then 

 frequently also the fibrous strand which may accompany it;— and secondly others, 

 which mark off the bundle-ring or cylinder from the surrounding tissue. The de- 

 signation of the two forms distinguished according to their position, follows obviously. 

 Inasmuch as in roots and stems, the axial vascular bundle, or the bundle-ring or 

 cylinder, corresponds to the primary plerome, while the surrounding parenchymatous 

 sheath answers to the innermost layer of the periblem, the name pleronie-sheath ^ is 

 appropriate for the latter, even in cases possibly occurring in which the name chosen 

 according to the primary differentiation of the meristem may not apply quite exactly. 



An internal sheath, occurring on the side of the bundle-ring next to the pith, 

 exists only in some species of Equisetum, mentioned at p. 122. 



The plerome-sheath of the stems of Phanerogams also lies outside the fibrous 

 strands accompanying the peripheral bundles of the cylinder, where such strands 

 are present. According to Van Tieghem ^ it is always separated from these or from 

 the pliloem-portions of the vascular bundles by a single or few-layered zone of 

 parenchyma, and this forms the continuation of the pericambium of the main root, 

 while the plerome-sheath itself is continued directly from the latter into the stem. 



Sheaths of the plerome and ring, as well as those of single vascular bundles, 

 appear in two principal forms, namely in the form of the eridodermis, or in that 

 of a single layer of cells, which agrees with the endodermis in the close lateral 

 connection of its elements, but is destitute of the characteristic structure of its walls, 

 being only distinguished from the surrounding tissue by less conspicuous differences 

 of cell-form, and by permanently containing abundant small starch-grains. From 

 its latter characteristic it has been called the starch-layer by Sachs. That the endo- 

 dermis is also often distinguished by containing abundant starch, was already stated 

 at p. 125. Other peculiarities of the cell-contents, as abundant tannin, characteristic 

 pigments, &c., also frequently occur in both cases. Apart from the close relation be- 

 tween the starch-ring and the endodermis already indicated by the above statements, 

 forms occur as to which it is doubtful whether the one or the other term is appro- 

 priate, and which are thus intermediate, e. g. the plerome-sheath in the stem of 



' Sachs, Textbook, p. 124, 2nd English edition. 

 ^ Ann. Sci. Nat. 5 ser. torn. 16, p. 112. 



