GROWTH 217 



used as a measure of the other (on which consult LIVINGSTON, 

 Botanical Gazette, 40, 1905, 178, and Science, 22, 1905, 146). 

 There is also the whole important subject of the toxicity of vari- 

 ous substances to growing roots, which has been earlier touched 

 upon (page 162), and on which there are some very important 

 contributions in recent volumes of the Botanical Gazette. Finally 

 there is the subject of growth from the minute or molecular 

 point of view, involving the nature of the growth of solid struc- 

 tures such as cell walls and starch grains, and the modes by 

 which they are modified after their formation, inclusive of sliding 

 growth. 



(&) Differentiation. 



Thus far under Growth we have been considering increase 

 of size only; now we turn to the other important phase of the 

 subject, differentiation of new parts. In reality this has itself 

 two phases: first, the formation of new cells, which usually pre- 

 cedes increase in size, and, second, the formation of permanent 

 tissues of special function after the increase of size has been 

 accomplished. Important though this subject is, it yet admits 

 of little practicable experiment, and the student must work it 

 up through the literature, in which he should give especial atten- 

 tion to the following matters: 



(a) The physiological relation of the cell to the organism, as to 



which is the physiological unit (compare WHITMAN on " The 

 Inadequacy of the Cell Theory," in the Journal of Mor- 

 phology, 8, 639). 



(b) The distinction in cell formation between differentiation and 



auxesis. (This may very readily be studied observationally 

 and experimentally, to which end the embryos and seedlings 

 of young succulents, especially Cactacece, are admirable.) 



(c) The significance and determinants of cell size in relation to 



body size. 

 ((f) The relations of the positions, directions, and numbers of cell 



divisions to adult form. 

 (e) Seat and nature of the control over these divisions, involving 



the mechanism of heredity. 

 (/) Mode and place of origin of new parts, leaf, stem, root, bud. 



