ANATOMY OF TISSUES. 83 
lating cells lying in rows or chains ending free in the cavity. 
These rows of assimilating cells spring from the tissue lying 
near the centre of the thallus. This tissue consists of elongated 
cells containing little or no chlorophyll, whose function is sup- 
posed to be that of conduction, as their long axis is parallel to 
that of the thallus. The rudimentary leaf occurs in these forms. 
It consists of a single layer of cells originating near the apex 
and growing out so as to cover and protect the apical cells. In 
this respect it has the true leaf function (that is, protection of 
the tender, growing cells of the apex) but in no other. It 
contains no chlorophyll, and its morphological rank as a leaf 
depends entirely on the hypothesis that the midrib of the 
thallus, with its accompanying wings, corresponds to the stem 
of the leafy plant. 
From this brief description it is seen that in this class of 
plants there is nothing resembling the pecular tissue which 
the fungous spore produces, except the short-lived protonema 
from which the real plant develops. Its tissues are formed in 
the same manner as those of the phanerogams, by division of 
the apical cell or cells, and subsequent growth of the new seg- 
ments. Leitgeb claims to be able to follow back this process in 
all cases to a single apical cell, but others think that in some 
instances there is a row of initial cells. There is a plainly 
marked epidermal system, but the separation into the other 
two, ground and vascular systems, is not yet accomplished. 
According to modern views on the relations of the different 
plant organs to each other, and to the homologues in the suc- 
cessive classes of plants, there occurs a distinct alternation of 
generations, beginning with the Bryophytes and extending 
through to the highest class of plants. In all cases the plant 
representing one of these generations is much smaller and sim- 
pler in structure than that of the other, and in nearly all cases 
this difference is so great that it is customary to consider the 
smaller plant only an appendage of the larger. In a work on 
