360 MESSRS. CTEIL WEST AND H. TAKEDA O.N" 



Anatomy and Histology of the Leaves. 



1, Epidermis. 



The epidermis of the leaf of Iso'etes japonica exhibits no special features, although the 

 outer wall is very highly differentiated for a water-plant. The outer wall is compara- 

 tively thick (4-8 ^\ and consists of an inner cellulose layer and an outer cuticularized 

 layer (PI. 36. fij^s. 22, 23). The outer surface of the epidermis is covered by a very 

 thin layer of cuticle. The primary membrane of the epidermal cells is very conspicuous, 

 particularly in the parting-wall between adjacent cells. On the inner side of the 

 primary membrane the cell-w^all consists chiefly of cellulose, but on the outer side the 

 thickening is more or less cuticularized. This well-marked differentiation of the outer 

 cell- wall into two distinct layers appears to have a mechanical, but not a physiological, 

 function. Moreover, this type of epidermis is not peculiar to Isoiites amongst water- 

 plants ; it also occurs, for instance, in the stem of Banunculus Jluitans. A comparison 

 of PI. 36. fig. 23 with PL 36. fig. 22 will show that the outer wall of the epidermis is 

 somewhat thicker in the 



amiria than in the basal region of the leaf. The cells of the 



ipidermis usually contain chlorophyll granules 



2. Stomal a. 



The apical portion of the leaf, unless it be completely submerged, is usually provided 

 with stomata. These stomata are of the gramineous type (PL 36. figs. 23, 24), and 

 possess no subsidiary cells. It appears, therefore, that a stoma-mother-cell divides but 

 once longitudinally and forms a stoma directly. The guard-cells are provided with a 

 conspicuous outer ridge. 



3. MesopJiyll. 



Large air-canals, which are always four in number, traverse the mesophyll. The 

 development of these lacunae, which are formed by the lysigenous degeneration of 



definite 



oups of cells in the very young leaf, has already been described 



detail by Farmer (14, p. 47) and by WHs on Smith (30, p. 230 et seq.). In these 

 lacun£B, transverse septa or diaphragms occur at intervals of a few millimetres. The 

 individual cells of these diaphragms have the stellate form commonly found in such 

 structures, especially amongst water-plants (PL 36. fig. 28). 



No distinct palisade tissue is formed, most of the mesophyll cells containing abundant 

 chlorophyll (PL 36. figs. 23, 41). The cells bordering the lacume towards the centre 

 of the leaf are always empty ; that portion of their wall which abuts on the lacuna is 

 slightly cuticularized (PL 36. fig. 41). 



Towards the base of the leaf the mesophyll cells contain very little chlorophyll, but 

 occasionally the presence of reserve starch can be detected in these cells. 



Whereas in some specimens of I. japonica the leaves possess hypodermal groups of 

 fibrous elements, in others no trace of these tissues can be detected. The presence or 

 absence of the hypodermal fibrous strands probably depends entirely upon the environ- 

 mental conditions to which the leaves are subjected at a very earlv stage in their 



