RECENT WORK, ETC. 467 



interrupted, and, where the leaves are crowded, the limits 

 of the stele as a whole may be difficult to recognise. This 

 however, though a difficulty, is not an objection. The 

 stele is none the less a distinct region in the stem because 

 it possesses prolongations into the leaves. 



In the internodes themselves, however, there is often a 

 much less definite outer limit to the stele in the stem than 

 in the root. The endodermis, which in the latter organ 

 forms so characteristic an inner boundary to the cortex, is 

 not always obvious in the stem, though often it is so. The 

 same remark applies to the pericycle. In the root this 

 layer was recognised by Nageli and Leitgeb (under the 

 name of pericambium) as long ago as 1867. The presence 

 of a corresponding zone in the stem was first established 

 by Van Tieghem in 1882 (6) and more fully demonstrated 

 by Morot in 1885 (7). The limit between pericycle and 

 cortex is of course difficult to recognise where the en- 

 dodermis is not specially differentiated. Still, by studying 

 the development, and by tracing the connection with the 

 tissues of the main root, the boundary can be sufficiently 

 well traced. In any case the occasional want of a sharp 

 limit to the stele is no argument against its recognition as 

 a distinct anatomical region, any more than the existence 

 of decurrent leaves is an argument against the morphologi- 

 cal distinction between leaf and stem. 



Where the vascular bundles bend out from the stem 

 into the leaf, they are accompanied by conjunctive tissue. 

 The name meristele is now applied by Van Tieghem to a 

 bundle or group of bundles entering a leaf, with their en- 

 veloping conjunctive tissue (8, p. 284). Thus we see that 

 the stelic tissue of the whole plant is continuous through all 

 its organs, root, stem, and leaf {cf. Strasburger, 3, p. 1 1 1). 



So far reference has only been made to the typical 

 stems of Phanerogams, in which there is a single central 

 cylinder forming the direct continuation of that of the main 

 root. This " monostelic " condition is constant in the 

 embryonic stem of all vascular plants. In many vascular 

 Cryptogams, however, and in two Dicotyledonous genera 

 {Gunnera and Primula, section Auricula) the cylinder 



