330 



Cavers : Notes on Yorkshire Bryophytes. 



scale consists of a triang-ular plate or ribbon of cells, the apex 

 of the triang-le bearing- a few long mucilag^e hairs. Further 

 back, these scales become withered, and none are to be found 

 on the older parts of the stem, their function being- simply that 

 of keeping the g-rowing-point moist. 



The leaves stand on the flattened wings of the stem (Fig. 3), 

 but sometimes encroach upon the thick median portion or 

 'midrib' or ma_v even extend across the middle line, in which 

 case the leaves of the opposite sides may join and form a 

 continuous lamella reaching across the whole plant. Each leaf 

 slopes forwards so as to overlap the next leaf in front, and 

 occasionally we find two or more of the leaves on the same side 

 of the plant becoming joined by a membranous outgrowth from 

 the stem-surface. The leaf consists of a single layer of cells, 

 forminer a ribbon which is often about 20 cells in maximum 



FiS- 3- — Transverse section tlirough the middle of the leaf-bearing region ot tlie 

 plant. L., leaves; M., mycorhiza; Rh., rhizoids, X 20. 



breadth and gradually narrows at either end ; the free margin 

 is usually even, but sometimes lobed, irregularly toothed, or 

 thrown into folds. The cells of the leaf are usually hexagonal 

 in surface view, and contain numerous large chlorophyll-grains ; 

 chlorophyll is also present, but sparingly and in smaller grains, 

 in the superficial cells of the stem. It is obvious that the 

 development of these leafy appendages on the upper surface of 

 the stem serves not only to increase the area of green assimi- 

 lating tissue but also to form numerous cavities in which water 

 can be retained. 



The growth of the shoot takes place by means of a single 

 apical cell, which has the form of a three-sided pyramid ; in 

 transverse sections through the growing-point this cell is seen 

 as an isosceles triangle, the shorter side being nearest to and 

 parallel with the ventral surface of the stem (Fig. 4, II.). From 



Naturalist, 



