So 
MORPHOLOGY OF TISSUES. 
which is not yet dififerentiated may be termed Primary Tissue, or, since its cells 
are always capable of division, Primary Meristem ^ 
In the present section a separate paragraph is devoted to each tissue-system, 
and a description will first be given of the various forms of cells and tissues which 
are constituents of all systems. Those 
forms which are peculiar to one or 
another system will be discussed in 
their proper place. 
(a) In reference to external form, 
the following cell-combinations may 
be enumerated: — 
(i) The term Tissue may be ap- 
plied par excellence to aggregations of 
similar cells which, without any well- 
defined external form, consist, in what- 
ever direction the section be made, of 
numbers of cells. Illustrations of such 
tissues occur in the greater number 
of the larger Fungi; the fundamental 
tissue of thick stems of Ferns and 
Monocotyledons is a parenchymatous 
tissue penetrated by other forms of 
tissue in the shape of strings. In 
Dicotyledons the pith especially comes 
under the same denomination ; it may 
also be found in the purest form in the 
flesh of succulent fruits. In stone- 
fruits, such as the peach, plum, cocoa- 
nut, &c., the stone consists of a 
sclerenchymatous tissue. It may also 
occur that forms of tissue which 
differ morphologically may be aggre- 
gated into a mass of tissue uniform 
in its physiological characters, as in 
the secondary wood of trees and the succulent tissue of tubers, such as the potato, 
dahlia, &c. 
(2) A row or Filament of Cells is composed of similar cells placed singly side by 
side or in rows ; but they are usually connected genetically. Isolated filaments of 
this character occur in the hyphae of Fungi and in many Algae, where they have 
arisen by transverse partition of an apical cell or by intercalary transverse division. 
In the higher plants we have abundant illustrations: — in the epidermal hairs, vessels^ 
laticiferous vessels, sieve-tubes, tannin-receptacles (as in the phloem of Phaseolus), &c. 
which are found in the interior of tissues. 
(3) A simple Layer of Cells results where similar cells are so united in one plane 
that the entire layer is only a single cell in thickness. Among Cryptogams it is not 
Fig. 67. — Transverse section of the stem of Sela<^inella inceqicali- 
folia. The outer layers of cortical have thick dark-coloured cell-walls ; 
the thinner-walled fundamental tissue envelopes three fibro-vascular 
bundles, separated from it by large intercellular spaces / (X 800). 
^ It may not be superfluous to remark that the pith and cortex are neither forms nor 
systems of tissue, but are altogether indefinite and undefinable; we speak, for example, of 
cortex in Thallophytes in quite a different sense to what we do in Vascular Plants ; the cortex of 
Monocotyledons is something different from that of Conifers and Dicotyledons; in the latter the 
cortex has quite a different signification in }oung and in older parts of stems. The same is the 
case with the pith. 
