362 MESSES. CYEIL WEST AND H. TAKEDA ON 



the cells of the ground-tissue surrounding the protopliloem undergo a considerable 

 change. They appear to undergo considerable elongation, and their walls, although 

 remaining cellulosic, become greatly thickened. These cells, according to Xruch, have 

 a mechanical function. The thick-walled cell figured by Janczewski (17, PI. 4. fig. 9) 

 and designated a sieve-tube, evidently represents one of these modified conjunctive 

 cells, which do not in the least resemble the sieve-tubes. 



The sieve-tubes are composed of elongated cells with more or less inclined terminal 

 walls (PI. 37. figs. 43, 44). Sieve-areas of the usual Cryptogamic type occur in abundance, 

 both on the lateral and terminal walls of these cells. They are often rendered very 

 conspicuous by a deposition of callus (PL 37. figs. 43, 44). 



Sieve-tubes of a similar nature are also found in the leaves of Iso'etes lacustris (PI. 37. 



fig. 45). 



In both species the sieve-tubes of the foliar bundle connect up directly with similar 

 elements of the primary and of the secondary phloem of the stem-stele (PI. 34. fig. 13 ; 

 PL 35. figs. 15, 21). 



The xylem of the foliar bundle consists of tracheids and parenchyma. Towards the 

 apex of the leaf the tracheids are very feebly developed, only a few metaxylem elements 

 being present besides the caaal formed by the disorganization of a single protoxylem 

 element. Here the position of the protoxylem is exarch (PL 37. fig. 46). The transverse 

 or slightly oblique terminal walls of the protoxylem elements are reticulately thickened, 

 tlie thickening being lignified (PL 37. figs. 49, 50). The primary membrane does not 

 break down ; hence the protoxylem canal consists of several superposed tracheids, with 

 almost horizontal terminal walls, and is not a trachea in the strict sense, since only the 

 annular thickenings of the lateral walls become disorganized. These protoxylem canals 

 are said to conduct water ; they must, therefore, retain their original function even 

 after the disorganization of the secondary thickening. 



In the adult leaf the protoxylem canal is always surrounded by a layer of regularly 

 arranged cells, the morphological nature of which has never been satisfactorily explained. 

 In I.japonica the radial walls of these regularly arranged cells, as well as that part of 

 the wall which abuts on the canal, are slightly thickened, highly refracti^-e, do not 

 dissolve in strong sulphuric acid, and present most of the characters usually associated 

 with an endodermis. Scott and Hill (28), and Janczewski (17), have regarded this row 

 of cells as a true endodermis, but we prefer to apply the term * pseudo-endodermis ' 

 to tliese (.'clls, since they difi'er from a typical endodermis both in origin and position. 

 In the lower region of the young leaf it is evident that the protoxylem elements are 

 surrounded by a distinct row of cells, some of which are typical metaxylem tracheids, 

 whilst others are parenchymatous (PL 37. fig. 47). Thus, in this part of the leaf some 

 of the * pseudo-endodermal ' cells are homologous with the water-conducting elements of 

 the foliar bundle. The fact that these cells occur in the centre of a vascular bundle and 

 generally surround only one of many tracheids , is sufficient evidence to show that they do 

 not constitute a true endodermis. Moreover, the slii?ht thickening and cuticularization 



of the radial walls of these cells are not features that occur only in endodermal cells : a 

 layer of ceils exhibiting all the characters generally associated with an endodermis 



