92 BRUCE—STRUCTURE AND FUNCTION 
secreting gland-cells separated from each other laterally, but 
united by their inner walls toa single central cell, which does 
not secrete, but gives a support to the secreting cells. 
More complex still is the condition where a third layer of cells 
has been developed between the outer gland-cells and the central 
row. This is the stage which most of the taller tentacles of 
Roridula have reached. ~ 
All these tentacles are simply epidermal out-growths of the 
nature of hairs. In none of them is there a central carrying- 
group of tracheids. But at the base of some of the longer 
tentacles, which are found just behind the apex of the leaf, there 
is sometimes a slight development of tracheid-tissue which 
passes into the pedicel for barely a quarter of its length, and into 
the tentacle terminating the leaf (and each of its side-branches 
in the case of Roridula dentata), a tracheid-strand is continued 
from the mid-rib into the centre of the stalk in its lower half 
or two-thirds (Fig. 30). It never reaches the head. 
These terminal tentacles are the most highly developed of all 
the tentacles found either on the leaf or stem, and may be com- 
pared directly with the tentacles found on the leaves of Drosera. 
The tentacle of Drosera may be considered to be derived from 
a type resembling that of Rorzdula. The tentacles in the two 
genera agree closely in length of pedicel and in length and 
diameter of head, but the vascular strand which traverses the 
pedicel and ends in a number of tracheid-cells occupying the 
centre of the head of the tentacle of Drosera is only fore- 
shadowed in the tentacle of Rortdula. The tracheid-cells in the 
centre of the head of Drosera represent the innermost row of 
cells and its surrounding layer in Roridula. 
The outer layer of gland-cells of the tentacle of Drosera 
arranged in two rows. an outer and an inner, are continuous with 
one cell-row of the pedicel, not with two, and correspond to the 
single row of gland-cells in Roriduda. 
The most striking structural difference between the tentacles 
is the presence in Drosera of the so-called middle layer. 
part from structure, its power of movement at once marks 
out the tentacle of Drosera as an organ showing advance 
upon that of Roridula, Darwin, in “lInsectivorous plants,” 
points out that the seat of movement in the tentacle of 
