CHAPTER II. 
MORPHOLOGY OF TISSUES. 
Sect. 12. Definition. — In the widest sense of the term, every aggregate of 
cells which obeys a common law of growth (usually however not uniform in its 
action) may be termed a Tissue. Aggregates of this kind may originate in different 
ways. The cells may be at first isolated ; subsequently during their growth they 
may come into contact, and so completely coalesce at the surfaces of their walls 
thit the boundary between them becomes indistinguishable. This happens, for 
example, in the sister-cells 
which have arisen by divi- 
sion in the mother-cells of 
Pediastrum, Coelastrum, and 
Hydrodictyon ; the sister- 
cells within the mother- cell 
have a 'creeping' motion 
which lasts for a consider- 
able time before they be- 
come united into a plate 
{Pediastrum) or into a sac- 
like hollow net {Hydrodic- 
tyoTi), and continue to in- 
crease as a tissue. In the 
same manner the sister- 
cells which arise in the em- 
bryo-sac of Phanerogams 
by free-cell-formation unite 
with one another and with 
the wall of the embryo-sac itself, continuing then to develop as a continuous tissue 
(the endosperm) and to increase by division. 
In Fungi and Lichens tissues originate by the juxtaposition and apical growth 
of slender filaments consisting of rows of cells (hyphae), and of different orders 
of branchlets from them; each filament has its own growth, increasing the 
number of its cells by division, and branches copiously; but this takes place in 
such a manner that the different hyphse undergo a similar development at definite 
Fig. <,A-—Pediastr2(m £^ra7ttclat2im (aiter A.. Bra.MW. X 400). a plate consisting of 
united cells ; at ^ the innermost layer of a cell-wall is protruding ; it encloses the 
daughter-cells resulting from division of the green protoplasm ; at t are various states of 
division of the cells ; sp the fissures in the already empty cell-walls ; B the inner lamella 
of the mother-cell-wall which has entirely escaped (greatly enlarged) ; b contains the 
daughter-cells i^, these are in active 'creeping' motion; C the same family of cells 
41 hours after its birth, 4 hours after the small cells have come to rest ; these have 
arranged themselves into a plate, which is already beginning to^ develop into one 
similar to A. 
