344 MANUAL OF BOTANY 



known as meristeles, pass outwards from it to the leaves. If 

 a section of such a stele be taken a little way behind the growing 

 point, there can be seen variously placed in it a number of col- 

 lections or groups of small cells, very well defined and marked 

 off conspicuously from the rest of the tissue. These are sec- 

 tions of strands of cells, called procambiitm strands, which a 

 little later are transformed into the vascular bundles. Great 

 differences appear in different steles in the composition and 

 arrangement of the latter, as already described. Between them 

 we find ground tissue ; and a layer of the same, the perioyele, 

 surrounds the whole stele externally, lying in contact with the 

 endodermis. The general arrangement of the intrastelar ground 

 tissue has already been described. 



I 

 I 



St 



Fig. 730, Polystelic stem ol Fern. St. Steles, sc. Bands of sclerenchyma. 

 Iiy. Hypodermal sclerenchyma. ep. Epidermis. 



In many stems this monostelic structure is not fomid. 

 Several separate steles sometimes exist, each containing 

 vascular bundles and surrounded bv layers of pericycle and 

 endodermis. Such stems are known as polystelic (fig. 730). 

 In some stems of this type the separate s,teles, or some of them, 

 are fused together for variable distances along their course. 

 The pericycle and endodermis then surround the mass formed 

 by such fusion, and we have the arrangement known as 

 gamostely. If the original plerome is not continued into a 

 single cylinder surrounded by an endodermis, but instead gives 

 rise to a number of strands, each consisting of a single vascular 



