VIII. WATER DEFICIT, OR UNSATISFIED HYDRATION 



CAPACITY. 



Much of the present confusion as to the nature, mechanism, and 

 course of growth is due to the fact that premature attempts have been 

 made to institute comparisons in measurements derived from organ- 

 isms fundamentally different. 



Thus, the growth of bacteria consists in the enlargement to a unit 

 size of cells high in proteins which become independent when this 

 volume is reached, and not being attached to other cells, then* presence 

 does not directly affect the rate of growth calculated upon their num- 

 ber, except in so far as their excretions in the medium may do so. 

 Furthermore, these, as well as organs which are submerged in the 

 liquid nutrient media, are in a condition approaching complete hydra- 

 tion in the complex of conditions in which they live, and these may 

 vary only within very narrow limits in many cases. 



Growth hi the higher complex plants implies the multiplication of 

 embryonic cells and the development of the greater number of them 

 into special static tissues to which the growing cells are inseparably 

 attached. Thus an internode, or a leaf, barely makes its appearance 

 before some of its cells have passed beyond the growing-stage and into 

 a condition of lessening change to a nearly static condition of maturity. 

 Enormous numbers of senescent and highly specialized cells are formed 

 through which the water-supply of the growing cells must pass and 

 which compete for the supply. The water-deficit, or the amount which 

 a cell-mass in the plant may take up, may therefore vary widely, as, 

 in addition to the internal changes, water is constantly being lost from 

 the surfaces by transpiration. An instrument attached to the terminal 

 portion of a plant records the changes in volume of cell-masses in all 

 of the possible stages between the apex and the base of the internode 

 or the point at which the stem may be fixed in the experimental pre- 

 parations. 



The tips of roots offer a generalized type of growth, which has been 

 the subject of more experimentation than any other part of the higher 

 plants. Even here the measurements invariably include the enlarge- 

 ments of masses of embryonic cells by imbibition alone, the inter- 

 mediate stage in which the increase in the size of the vacuole doubtless 

 plays an important role, to a final stage in which imbibition again 

 may be the only force of distension. Only in individual unicellular 

 organisms and in such structures as pollen-tubes may growth-enlarge- 

 ments be dealt with to the exclusion of variations of mature tissues. 



Again, growth is a resultant of the play of molecular forces in sur- 

 face tensions and of a series of metabolic transformations, in which 

 features notable differentiations are found in the groups of organisms. 



92 



