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 

 that 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 

 Pediasirum, Ccelastrum, and 

 Hydrodiciyon ; 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 

 {Pediasirwri) or into a sac- 

 like hollow net {Hydrodtc- 

 tyon), 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 hyphae undergo a similar development at definite 



Fig. e^/^—Pediastrum granulatmn (after A. Braun, X 400). A a plate consisting of 

 united cells ; at g 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 ; R 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 

 4i 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. 



