398 • THE FORMS OF TISSUES [ch. 



texture and minute sculpture or pattern, it comes to pass, through 

 the laws of surface-tension and the principles of the geometry of 

 position, that a very small number of diagrammatic figures will 

 sufficiently represent the outward forms of all the tetraspores, 

 four-celled pollen-grains, and other four-celled aggregates which 

 are known or are even capable of existence. 



We have been dealing hitherto (save for some slight exceptions) 

 with the partitioning of cells on the assumption that the system 

 either remains unaltered in size or else that growth has proceeded 

 uniformly in all directions. But we extend the scope of our 

 enquiry very greatly when we begin to deal with unequal growth, 

 with growth, that is to say, which produces a greater extension 

 along some one axis than another. And here we come close in 

 touch with that great and still (as I think) insufficiently appreciated 

 generahsation of Sachs, that the manner in which the cells divide 

 is the result, and not the cause, of the form of the dividing 

 structure : that the form of the mass is caused by its growth 

 as a whole, and is not a resultant of the growth of the 

 cells individually considered*. Such asymmetry of growth 

 may be easily imagined, and may conceivably arise from a 

 variety of causes. In any individual cell, for instance, it may 

 arise from molecular asymmetry of the structure of the cell-wall, 

 giving it greater rigidity in one direction than another, while all 

 the while the hydrostatic pressure within the cell remains constant 

 and uniform. In an aggregate of cells, it may very well arise 

 from a greater chemical, or osmotic, activity in one than another, 

 leading to a locahsed increase in the fluid pressure, and to a 

 corresponding bulge over a certain area of the external surface. 

 It might conceivably occur as a direct result of the preceding 

 cell-divisions, when these are such as to produce many peripheral 

 or concentric walls in one part and few or none in another, with 

 the obvious result of strengthening the common boundary wall 

 and resisting the outward pressure of growth in parts where the 

 former is the case; that is to say, in our dividing quadrant, if 



* Sa,chs,Pflanzenphysiologie{Vorlesiingxxiv),'lSS2; cf. Rauber, NeueGrundlage 

 zui- Kenntniss der Zelle, Morphol. Jahrb. vin, p. 303 seq., 1883; E. B. Wilson, 

 Cell-lineage of Nereis, Joiirn. of Morphology, vi, p. 448, 1892, etc. 



