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ISOETES JAPONICA, A. BE. 



363 



completely surrounds the glossopodium of tlie ligule (PI. 36. fig. 42). In Elodea^ and in 

 some species of Fotamogetony the lysigenous protoxylem canal is surrounded hy a row 

 of regularly arranged cells, the radial walls of which, however, do not appear to he 

 cuticularized. 



As many as five protoxylem canals may he found in the supra-ligular region of the 

 leaf. They are invariahly formed hy the disorganization of protoxylem elements, and 

 are always surrounded ^}j a row of ^ pseudo-en dodermal* cells. 



In the ligular region of the leaf a marked increase in the numher of metaxylem 

 elements is found, hut the position of the protoxylem remains exarch (PL 37. fig. 48). 

 In the sporangial region of the leaf, and lower down also, a few metaxylem elements 

 are differentiated on the phloem side of the protoxylem, while many are present on the 

 opposite side; hence, in this region of the leaf, the protosylem is mesarcli. 



The metaxylem of the laminar portion consists of elongated spiral or suhreticulate 

 tracheids with very oblique terminal walls (PI. 37. fig. 51). In the sporangial region 

 the tracheids are somewhat wider, have less oblique terminal walls, and pursue a sinuous 

 course. In that part of the leaf-trace which traverses the cortex of the stem the xylem 

 is better developed and more compact than in the leaf itself. Sooner or later, however, 

 the conducting tissues become more or less stretched as a result of the formation of fresh 

 secondary tissues {i. e., phloem and cortex) hy the cambium, and of the increase in size of 

 individual cells of the cortex of the stem. For this reason the tracheidal elements of an 

 old leaf-trace embedded in the cortex become greatly distorted. On the other hand, the 

 xylem parenchyma, which retains its meristematic condition for a considerable period, 

 is able to adapt itself to the changing proportions of the surrounding cortical tissue hy 

 dividing both transversely and longitudinally (PI. 37. fig. 54). 



The phloem is subjected to a similar strain, but its complete destruction is somewhat 

 delayed by repeated divisions of the phloem-parenchyma. As a result of this cell-division 

 the leaf -trace traversing the cortex becomes much wider with age (PL 37. fig. 54). 



5. lAgule. 



The earlier stages in the development of the ligule of I japonica agree with those 

 described and figured for /. ecUnospora and /. Bngelmanni by Wilson Smith (30, p. 232, 

 PL 14. figs. 12-19). The ligule arises from a single epidermal cell of the very young leaf 

 (PL 36. fig. 35). This cell contains very dense protoplasm, and rapidly increases in size ; 

 it then divides by a periclinal wall into two daughter- cells (PL 36. fig. 36), each of which 

 immediately divides again by periclinal walls (PL 36. fig. 87). 



The apical region of the young ligule divides most vigorously, and ultimately gives 

 rise to the laminar portion, whilst, by the subsequent growth of the basal region, 

 the glossopodium and ligular sheath are formed (PL 36. figs. 38-40; also cf. PL 39. 



figs. 66, 68). 



The form of the mature ligule is ovate-lanceolate, cordate at the base, with a lacerate 

 margin (PL 36. figs. 25-27, 33). The central region of the ligule is many cells in 

 thickness, but towards the margin it is never more than one cell in thickness. The 

 laceration of the margin is brought about by a very peculiar method of sliding growth. 



