DIV. I 



MORPHOLOGY 



35 



protoplast and is not itself regarded as living. Many plants 

 commence their development as naked protoplasts, e.g. swarm-spores 

 or egg- cells. These cells, before developing further and dividing, 

 secrete a thin cell wall clothing the surface. In cell division, as has 

 already been described, a partition wall is usually formed between the 

 new cells so that each protoplast remains enclosed by a cell wall. 



The form of cells is usually dependent on the cell wall, for the 

 naked protoplast behaves like a fluid drop. The relatively small and 

 uniformly shaped meristematic cells attain their ultimate size and 

 special shapes by the growth in surface of their walls. This growth 

 is sometimes the same all round, and at other times is limited to 



-B 



tne tip or an 



a girdle -like zone, or some other 



circumscribed region. It comes 



FIG. 29. A , Spherical stalked cell 

 of Saprolegnia with circular 

 pits in the wall. B, One pit 

 of this in optical section more 

 highly magnified. 



FIG. 30. Sclerotic cell from the shell of a walnut 

 showing stratification of the wall and branched 

 pits. The canals of some of these pass ob- 

 liquely out of the plane of section. (ROTHERT, 

 adapted from RELNKE.) 



about as a result of the stretching and sometimes the rupture of the 

 wall and the secretion and deposit of new cell-wall substance by the 

 protoplast (GROWTH BY APPOSITION), or else by the insertion of new 

 material between the particles of the existing wall (GROWTH BY 

 INTUSSUSCEPTION). 



The cell wall serves to protect and also to give rigidity to the 

 protoplast. This is attained both by the tension of the membrane 

 (TURGOR, cf. p. 225) and by the growth in thickness of the cell wall. 

 The thin and structureless walls become as a rule thickened either 

 uniformly or so that parts remain relatively thin, while others grow 

 in thickness. In many cells the whole extent of the wall is thickened 

 with the exception of small circular, elliptical, or spindle-shaped 

 areas which form the PITS. These appear in the thickened wall as 



