PHYSIOLOGY 



483 



the large sap-cavity which occupies the greater part of the cell. 

 The stretching of the cell, which is a very important factor in its 

 enlargement, is not the only factor, as there is also a large amount of 

 material added to the thickness of the cell-wall. This growth in 

 bulk of the cell-wall is due in part to the addition of new layers 

 of cellulose from within by " apposition," and probably, also, to 

 " intussusception," or the insertion of new particles of cellulose 

 between those in the wall. Whether these particles are, chemi- 

 cally speaking, molecules, or larger bodies (Micellae), each composed 

 of masses of molecules, is not certain. 



The growth of the cell may be mainly longitudinal, as in most 

 stems ; or it may be lateral, as in flat organs like leaves. 



The last phase in the development of the cell is its assumption of 

 its specific character as part of a special tissue. The change in form, 

 and secondary changes in the cell-wall, such as thickenings and 

 chemical changes, found in wood-cells and other highly specialized 

 tissue-elements, illustrate this. Such cells once formed, are, as a rule, 

 either dead, or are incapable of further growth and division. 



There are, then, three phases in the development of the tissues, 

 cell-division, growth, and differenti- 

 ation. ' 



IRRITABILITY (PFEFFEK, 

 pp. 10-23) 



In discussing the nature of proto- 

 plasm, one of its most striking char- 

 acteristics noted was irritability, i.e. 

 sensitiveness to the various forces 

 and substances which compose its 

 environment. Indeed, all the vital 

 processes are indissolubly associated 

 with this property of protoplasm. 

 Among its most evident manifesta- 

 tions are the various forms of move- 

 ment characteristic of all living 

 organisms. An exception must be 

 made in the case of movements 

 exhibited by dead structures, such as 



the hygroscopic movements shown Fm - 7 - T Hygroscopic movements 

 , -r, shown by the awn of Erodium. 



by the elaters ot Equisetum or A, dry; B, wet(x2). 



Liverworts, the opening of sporan- 

 gia, the movements of the awns of Erodium (Fig. 457) and Stipa, 

 bursting of capsules in Viola or Impatiens, etc. These are purely 

 mechanical, and quite unconnected with the irritability of the pro- 

 toplasm. 



