102 Oliver, — On the Structure , Development , 
near the edges of the leaves. These, from the arrangement of 
the guard-cells, must remain permanently open. 
In Figs. 64 and 65, a surface view and vertical section of 
one of these four-celled glands is given. In 64 its pedicel (p.) is 
supposed to be seen through, and is dotted in. In 65 the exact 
relations of the parts are given. The development — which 
can be easily traced in young leaves — is simple. A small cell 
projects from the epidermis, and is divided by a horizontal 
wall ; the lower cell constitutes the pedicel, the upper enlarges 
and divides cross-wise into four cells. In some cases, especially 
on the stem and flower-stalks, the cells of the gland grow out 
to a great length, forming a distinctly quadrifid gland. The 
possession of these glands is a decidedly Pedalinaceous 
character — they are formed in great quantities in Pedalium , 
Pretrea , etc. In these latter genera they secrete mucilage in 
quantities, preventing the plant from drying up. In Fig. 66 
a and b are given views of such a gland from Pretrea for com- 
parison with Figs. 64 and 65. The concentric layers within 
the cuticle swell and break down into great quantities of 
mucilage when the gland is placed in water. The shaded rod 
at the inmost angle of each cell represents the collapsed proto- 
plasm of the cell. That Trapella , an aquatic plant, should need 
special mucilage-glands to prevent its being dried up is im- 
probable. It may be that their presence in our plant is due 
to the retention of an ancestral character. In any case, the 
possession of such glands by many genera of Pedalineae is 
a point worth noting, though in determining affinities it is 
possible to place too great belief in anatomical characters. 
Another very interesting point in the leaf-anatomy of 
Trapella is the possession of water-glands by both floating and 
submerged leaves. These are situated in the small incisions 
of the leaf-margin (one in each),, as in Saxifraga crustata. 
Fig. 58 is a vertical section through a water-gland in one of 
the floating leaves. The string of tracheides may be seen 
ending in a mass of small-celled tissue, the epithem (e) well- 
marked off from the surrounding mesophyll. Above the 
water-gland is the water-pore ( zvp , ). In Fig. 59 this is seen 
