70 FIRST FORMS OF VEGETATION. 
to form apparently a continuous whole. They are 
disposed either in a spiral which turns from left to 
right or from right to left. They overlap each 
other in two ways; either each leaf covers with its 
lower edge a little of the leaf below it, in which 
case the leaves are called succubous, or each leaf 
overlaps a little of the base of the leaf above it, in 
which case the leaves are called zzcudous. In their 
shape there is a marvellous diversity, being fre- 
quently deeply toothed or bi-lobed ; and the ar- 
rangement and form of the cells is so exquisitely 
beautiful in almost all the species, that no more 
pleasing objects can be mounted for the micro- 
scope. The grains of chlorophyll often exhibit in 
them, as in the leaves of the true mosses, appa- 
rently spontaneous movements under the influence 
of light. If kept in the dark for several days the 
cells present the appearance of a green net-work, 
between the meshes of which is a clear transparent 
ground. All the grains of chlorophyll are applied to 
the walls which separate the cells from one another. 
If placed again in the sunshine, or even under the 
influence of artificial light, the grains change their 
position from the lateral to the superficial walls— 
where, however, they do not remain absolutely im- 
moveable, but continually approach and separate 
from one another for upwards of a quarter of an 
hour. If again darkened they leave their new posi- 
