﻿84 DEVELOPMENT OF STEM AND LEAVES OF PHYSIOTIUM GIGANTEUM. 



adapted, both by its shell-like form and its position on the upright 

 stem, for catching and holding water (see fig. 2), which, owing to 

 its somewhat oblique posture, will naturally flow towards the 

 auricle. It is prevented from running off the lobe by the lower 

 (.•■]_'■ - - ; - ".y '. ■ - . . - ■ ' " ■ • ■ 



water may collect, and leading to the mouth of the auricle. The 

 curved edge of the lobe is continued by a projection into the mouth, 

 the upper edge of which is raised to prevent the water escaping at 

 the junction of the two members, while a depression below forms a 

 channel which ensures the entrance of the water into the auricle 

 (%. 7). 



The development of the upper lobe of the leaf presents no 

 special features, except that in a young stage it hangs three 

 mucilage papillae over the apex of the stem (fig. 9), remains of 

 which can be seen in the mature lobe in the interruptions of the 

 otherwise regular outline and boundary layer. 



The development of the auricle is naturally much more com- 

 plex (fig. 8). When the rudiment consists of a few cells only, it 

 rapidly becomes rolled in until it has assumed a slipper-like form, 

 which is evidently due to localised growth similar to that occurring 

 in the bladders of Utricular in. The apex now bifurcates (indicated 

 in the youngest auricle in fig. 8). One of the new apices grows 

 directly, by a two-sided apical cell, into the clapper of the valve ; 

 the other divides into three well-marked apical cells, and these 

 give off a few segments, the products of which subsequently grow 

 more rapidly than the apical cells themselves, filling up the gaps 

 between them, and an approximately semicircular plate of cells 

 (the rigid flap) results. Meanwhile the curtain lias been formed by 

 the circular depression of the region behind the growing valve-flaps, 

 and the collar formed by the folding of the tissue around. The 

 stem-tissue from which the auricle arises has become, immediately 

 below the mouth, an annular merismatic zone, while that of the 

 upper lobe has become connected with it, so that the two now arise 

 together (figs. 5 and 6). 



Both the upper lobe and the auricle (including the rigid flap of 

 the valve) have a marked cellulose thickening of the wall (fig. 9). 

 The large pits enable the water to pass easily from cell to cell, 

 while, as may be seen from the ready crumpling of their lamella, 

 they are no doubt useful in enabling the foliage, especially the 

 auricle, to contract on drying, which decreases the evaporating 

 surface, and hinders the entrance of air into the cavity of the 

 auricle. There is no special provision, as might have been 

 expected, for the entrance of water from the reservoir into the cells 

 of the auricle, no pits whatever occurring on the walls immediately 

 surrounding the cavity (fig. 10). The wall-thickening, as so often 

 occurs in the alteration of cell- walls, begins in the corners, and at 

 an early stage presents an elegant appearance, owing to the trilobed 

 structure first produced (fig. 11). 



There is no differentiation in the tissue of the mature stem, 

 except that the outer cells are rather more strongly thickened than 

 those within ; the thickening is much like that of the leaves, 



