CHAPTER II. 



MORPHOLOGY OF TISSUES. 



Sect. 12. Deflnition. — In the widest sense 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 

 concerned may be at first isolated, subsequently during their growth they may come 

 into contact, and become so completely united at the surfaces of contact of their walls 



that the boundary surface 

 between them becomes in- 

 distinguishable. This hap- 

 pens, e.g. in the sister-cells 

 which have arisen by divi- 

 sion in the mother-cells of 

 Pediastrum,Coelastrum, and 

 Hydrodictyon ; the sister- 

 cells show in these cases 

 within the mother -cell a 

 ' creeping ' motion which 

 lasts for a considerable 

 time before they become 

 connected into a surface 

 (Pediastrum), or in the form 

 of a sac-hke hollow net 

 (Hydrodictyon), and form 

 by their growth a tissue. 

 In the same manner the 

 sister-cells (endosperm) which arise in the embryo-sac of Phanerogams by free-cell- 

 formation, unite with one another and with the wall of the embryo-sac itself, con- 

 tinuing then to develop as a continuous tissue and to increase by division. 



In Fungi and Lichens the formation of tissue originates by the apical growth 

 of juxtaposed thin filaments consisting of rows of cells (the hyphse), and different 

 orders of branchlets of them ; each filament grows by itself, 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 spots on the whole 

 body of the Fungus or Lichen ; thus arise surfaces, strings, hollow structures, &c., 



Fig. 'i^— Pediastrum granulaUiin (after A. Braun) (X400). A a disc consisting of 

 cells grown together ; at g the innermost layer of a cell-wall is protruding ; it contains 

 the daughter-cell* 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 (g), these are in active creeping motion ; C the same family of cells 

 4j hours after its birth, 4 hours after the small cells have come to rest ; these have 

 arranged themselves into a disc, which is already beginning to develop into one 

 similar to A. 



