THE ANGIOSPERMAE : STEMS 



883 



It is evident that the stelar sheath in the stem is by no means uniform, 

 and Strasburger has coined the useful name phloeoterma to apply to the 

 innermost layer of the cortex, which must be recognized to be an important 

 anatomical boundary, whether or not it has the character of a true endodermis. 



6. The Primary Vascular Tissues. Everything within the endodermis is 

 denominated the stele. The question of the applicability of this concept, 

 derived from the study of the stem in the Vascular Cryptogams, to the stems 

 of the Spermatophyta, we have discussed previously. It is at any rate certain 

 that in the absence of an endodermis, and particularly in some Mono- 

 cotyledons, it is impossible to give it the precise connotation which it has, for 

 example, in the Ferns ; but it is a handy descriptive term and we shall continue 



Pericyclic fibres 

 Phloem 



Interfascicular 

 cambium 



Metaxylem 

 Protoxylem 



Fig. 870. — Helianthus rigidus. Transverse section of 

 stem showing isolated groups of pericyclic fibres 

 opposite the primary vascular bundles. 



to apply it, without prejudice, to the complex of tissues lying within the 

 cortex and of which the vascular tissues form so important a part. Its outer 

 boundary is usually called the pericycle. 



The pericycle in the stem is very different from the meristematic zone 

 which goes by the same name in the root. The idea of a pericycle is that of a 

 cell layer formed from the outer zone of the procambium, within the endo- 

 dermis and immediately surrounding the phloem. This position in the 

 stems of Dicotyledons is often occupied by a zone of sclerotic fibres or of 

 sclereids, sometimes continuous, sometimes broken up into groups which lie 

 outside the vascular bundles (Fig. 870). At the present day it is doubtful 

 whether this fibrous zone is really distinct in all cases from the phloem, for 

 it has been shown that it may arise from protophloem in which the sieve tubes 

 and other soft elements have been obliterated and only phloem fibres remain. 

 Whether this is always so cannot yet be stated. The vascular bundles of 

 Monocotyledons, whether they have an individual endodermis or not, are 



