THE STEUGTURE OF THE STEM 375 



stele is clothed by pericycle and endodermis. The latter layer 

 differs from the endodermis of the monosteho stem in that the 

 origin of part of it at any rate takes place in the plerome and 

 not the periblem. Each stele contains usually more than one 

 vascular bundle (fig. 759). The separate xylems of these steles 

 are fused together, so that there is no pith. The separate steles 

 may be irregularly disposed through the thickness of the stem, 

 or they may be arranged in a more or less ring-like manner. In 

 the latter case the central tissue must not be confused with the 

 pith of a monostelic stem. The separate steles frequently ana- 

 stomose with each other, forming an irregular network which can 

 readily be seen after destruction of the soft tissues by maceration. 

 Two varieties of the arrangement occur. In the first, which is 

 known as dialystely, the steles remain for the most part inde- 

 pendent, so that a transverse section shows them separate or 

 fused two or three together (fig. 760) . In the second the steles, are 

 arranged in a ring, and are close together. Many of them fuse 

 together laterally, giving the appearance of an almost complete 

 ring of vascular tissue, simulating the appearance often of the 

 ring of collateral bundles of the dicotyledonous type of monostelic 

 stem. As they fuse, the endodermis 

 and pericycle on their lateral faces 

 disappear, and the parts of those 

 layers behind and in front of them 

 become continuous. The ring of 

 tissue can be distinguished from the 

 ring of the dicotyledonous stem by 

 the presence of internal endodermis, 

 pericycle, and phloem layers (fig. 761). 

 This mode of arrangement, known 

 as gamostely, is met with in certain 

 Perns, especially Marsilea, and in some 

 species of Auricula among Phanero- -P*?- ^62. erowing point of 



cjjv.,vyi^B " o shoot of Equisetum arcense. 



gams. X 250. a. Apical cell. 



The conjunctive tissue in both ^^^^^^ ^''^"^"^ 

 polystelio and schizostelio stems is 



partly derived from the periblem and partly from the plerome. 

 Their growing points are generally furnished with an apical 

 cell (fig. 762, a), though sometimes they have a small-celled 

 meristem. 



