604 



SENSORY SYSTEM 



tained in the so-called " starch-sheath " [the endoderniis]. 30 ' 5 Formerly, 

 this layer was regarded either as a carbohydrate-conducting tissue 

 (Sachs), or as a local storage-organ (Heine), serving for the nutrition 

 of the adjacent fibrous cylinder or circle of mechanical strands during 

 its early development. 



Morphologically considered, a typical single-layered starch-sheath 

 represents the innermost layer of the primary cortex (the phloeoterma 



Fig. 249. 



Part of a T.S. through a scape of Aram ternalum, .showing a erescentie group of 

 statocysts interiiolated between a collenchynia strand and a vascular bundle. 



of Strasburger). Externally, therefore, it abuts against the cortical 

 parenchyma, while internally it is contiguous to the vascular cylinder 

 or circle of conducting strands, or to the fibrous tissue associated with 

 the mestome (Fig. 248). In many plants, each individual vascular 

 bundle is enclosed in a separate starch-sheath. 



Closely related to the above-described normal type of structure, 

 is the condition in which the starch-sheath is interrupted by individual 

 cells devoid of starch, or by a whole series of such elements. In 

 Urtica dioica the disruption of the starch-sheath has progressed still 

 further, the leaf-traces being subtended by solitary statocysts, or rather 

 by longitudinal rows of these cells. Not infrequently the continuous 

 "starch-sheath" is replaced by sickle-shaped groups of starch-containing 

 cells, which are associated with the leptome- or hadrome-strands. Such 



