90 < МЕ. Н. М. WARD ON THE STRUCTURE, DEVELOPMENT, 
Besides the plate of nearly equal thin-walled elongated cells, forming the main body of 
the thallus, there are to be observed certain structures developed from the surfaces, which 
may be conveniently regarded as appendages of the nature (morphologically) of tri- 
chomes. These are cellular outgrowths, differing in size, shape, contents, and functions, 
each of which must be looked upon as a modification of the ordinary thallus-cell. They 
are either simple enlargements of single cells, the contents of which pass over into 
zoospore-like motile bodies; or they are prolongations downwards, fastening the thallus 
to its substratum, like the rhizines in ordinary Lichens, or stiff processes projecting out- 
wards and upwards as free “ hairs." Certain modified forms of the latter also subserve 
reproduction. 
The downward prolongations, ог rhizoids *, аз they шау be conveniently termed, are 
simple outgrowths of individual thallus-cells, which become interposed between the 
thallus-plate and its substratum, growing irregularly, but chiefly along the plane of the 
epidermis, and branching like very simple lobes of the Alga. Ifa flourishing thallus be 
carefully removed by insinuating the blade of a thin razor between it and the surface 
of the leaf, the rhizoids may be distinguished, on looking at the lower surface of Ше 
thallus under the microscope, as a series of sinuous, often compressed, irregularly 
ramifying, few-celled processes, at a focus higher than that in which the body of the 
thallus is distinct (Pl. XIX. figs. 17 & 18). Each of these outgrowths is a diverti- 
culum from the lower wall of a thallus-cell (fig. 12). If, after removing as above, the 
epidermis on which the rhizoids were spread be now observed, the outlines of these may 
Ре seen, as if imprinted as a pattern on the cuticle of the leaf. This is an expression of 
the fact that the rhizoids cling quite close to the cuticle, their walls becoming fused 
more or less completely with its substance—a process which is carried even further in | 
some cases, as on Citrus-leaves, described below. As a rule, the rhizoids contain the pale, 
diffused, chlorophyll-like colouring matter, and few orange-red globules but no nucleus ; 
sometimes, however, especially in an older thallus, the rhizoids are filled with orange-red 
globules, or become colourless, and are so closely fused with the epidermis below that they 
cannot be separated without rupture. They may, apparently, be produced from any cell 
of the thallus, but, though crowded in some cases, may be considerably scattered in others. 
It is not common to find the rhizoids developed from the walls of the cells containing the 
zoospore-like bodies; but even this occurs, as shown in Pl. XIX. fig. 19 а, where three 
short rhizoids have been produced by such a cell, or its immediate neighbour, and are 
closely applied to the cell-wall. In no instance have I observed any of these outgrowths 
piercing the epidermis of the leaf on which the thallus vegetates. 
At fig. 19 is a carefully drawn example of such a rhizoid as developed from a marginal 
cell; its walls (as also the external walls of the cells at the edge of the thallus) are 
thickened as by a cuticular and striated increase of their substance. This example paves 
the way for the consideration of the processes sent forth by the thallus-cells into the air, 
and which may be called trichomes, or simply hairs. 
If the thallus be examined by means of vertical sections taken through the whole leaf 
* Since these anchoring bodies belong to the Alga alone, they may be more accurately compared to the processes 
at the base of @doyonium, Bulbochete, «с. 
