VARIOUS ACTIVITIES 3 



the surface area is a limiting factor. One of three things 

 now is possible : the cell may remain as it is, a permanent 

 tissue element ; it may develop further, using up its own 

 contents either entirely or in part in fitting itself for another 

 function, water transport for instance; or it may divide and 

 by so doing increase its surface area in relation to its volume, 

 in which case the cycle may restart. Growth thus can be 

 interpreted in terms of physical chemistry : the first possibility 

 mentioned hardly requires contemplation, since nothing is 

 easier to do than nothing. The third proposition is less 

 easy to understand ; the second is a mystery. For instance, 

 why should the daughter of a merismatic cell develop into a 

 phlcem element if it be cut off on the one side of its parent 

 and into a xylem element if it be born on the other? Is 

 it due to some subtle influence or stimulus which has its 

 origin in the adjacent structural elements; or is it due to 

 some quality in the cell itself, an heredital predetermination ; 

 or is it due to some obscure colloidal property comparable 

 to the Liesegang phenomenon ? * 



To these questions there are no real answers ; the facts 

 must be accepted, their explanation must be left to the future. 



The embryo grows and develops into the autotrophic 

 organism of a form and structure determined by its conditions 

 of life and by its ancestry and exhibiting those actions and 

 reactions commonly associated with the higher plants. The 

 shoots and roots circumnutate and respond to various stimuli, 

 gravity and light being the most obvious. With respect to 

 circumnutative and other autonomous movements, these may 

 be explained by such conceptions as rectipetality and associ- 

 ated engrams ; whilst in explanation of tropisms various 

 mechanistic hypotheses have been formulated ; some chemical, 

 Czapek's explanation of gravitational stimulus of roots,f for 

 instance ; others physical, the statolith theory, for example. 



The highly organized root system by means of its root 

 hairs takes up raw material by osmosis in the form of water 

 and its dissolved salts; in special cases, possibly in all, the 

 osmotic strength of the cell sap of the root hairs is continu- 

 ously adapted and is nicely adjusted to the osmotic strength 

 of the soil water. From the root hairs water is passed on 

 * See Vol. I., p. 301. f Vol. I., p. 361. 



I * 



