336 SCIENCE PROGRESS 



this gap (indicated by a X), the central parenchymatous tissue 

 may be termed pith and may be considered to be stelar in 

 origin. But above the gap (indicated by a circle) the central 

 tissue would be morphologically distinct from the stele on 

 account of the presence of the internal endodermis. Thus, 

 in this upper region the pith would be absent unless there 

 happened to be a band of parenchyma between the inner 

 endodermis and the vascular tissue. On the other hand, 

 another plant might be precisely similar excepting for the 

 absence of the internal endodermis (fig. 5, b). So that, in this 

 case, the pith at the base would be morphologically the same 

 tissue as the central parenchymatous mass above the first 

 leaf-gap. 



On account of these facts, many have discarded the view that 

 the endodermis is a tissue possessing any morphological signi- 

 ficance ; but, on the other hand, the older opinion is still held by 

 some, for very little is known regarding the physiological role of 

 the endodermis, and in the majority of cases the endodermis is 

 present in the young stages, even when absent from the adult 

 plants, having disappeared through reduction (Jeffrey). 



It is possible that the endodermis may function as an air- 

 tight layer of cells around the vascular tissue, separating it from 

 air-containing intercellular spaces, and so preventing the access 

 of air into the tracheae. 



Some such provision as this is apparently very necessary 

 in the Ferns, for example, as, very generally, no secondary 

 thickening takes place, and thus a tracheid rendered more or 

 less functionless by the entry of air cannot be replaced. For this 

 reason the endodermis completely surrounds the vascular tissue. 



Drabble (17, p. 66) remarks that "It is with the xylem portion 

 of the bundle that this [the above] function of the endodermis 

 is generally assumed to be associated. This, however, is not 

 very clear. In roots the endodermis may be provided with 

 non-cuticularised ' passage-cells.' These cells occur opposite 

 the xylem groups, while the cuticularised elements are usually 

 continuous over the phloem regions. The frequent reduction 

 of xylem in aquatic plants is not necessarily, or even generally, 

 associated with the reduction in the endodermis." 



He points out instances of the absence of the endodermis 

 where, presumably, it should be present, and suggests " that the 

 endodermis may find its function rather as a phloem sheath than 



