XII. WATER-CONTENT. DRY WEIGHT, AND OTHER 

 GENERAL CONSIDERATIONS. 



Two different types of organs or shoots with respect to the variations 

 in the water-content and dry weight are recognizable in the material 

 which has served for studies in growth as described in this volume and 

 in the work of other writers. The commoner types of woody stems, 

 of thin leaves, and of the organs of the greater number of the higher 

 plants undergo a development which terminates in a mature stage in 

 which the proportion of solid material is very much higher than that 

 found in younger material. A parallel procedure is the prevalent one 

 in the tissues of the higher animals. Thus, by way of illustration, 

 Donaldson found that the proportion of water in the bodies of mammals 

 diminishes with age, and Hatai has shown that the percentage of water 

 is an indicator of phases of chemical alteration in the composition of 

 the body.^ 



Growth and differentiation of cell-masses into specialized tissues is 

 not inseparably connected with increases in dry weight, however, as 

 has been demonstrated by studies of the growth of frog larvae^ in the 

 earUer stages, and it is highly probable that similar phenomena are 

 prevalent in the fleshy fungi and other lower forms of plants. 



The distinction between the two kinds of growth has not been made 

 previously in studies of plants, and the matter was finally taken into 

 consideration in the experiments late in 1918. Stems of Helianthus 

 and pods of Phaseolus illustrate the kind of material in which dry 

 weight increases with age, upon which the greater part of all studies 

 in growth have been carried out. 



Etiolated plants furnish examples of growth with a diminished 

 increase in dry weight. Chief interest attaches to plants which nor- 

 mally show such action, and the most striking illustrations are 

 furnished by the organs of succulent plants and by fruits. The 

 relative amount of solid material in the flattened joints of Opuntia does 

 not increase with the course of development toward maturity, and 

 joints which have reached full size may contain over 91 per cent of 

 water. Secondary thickening, especially that which results from 

 branching and the development of additional fibrovascular tissue, 

 may cause an added amount to be formed. The proportion of dried 

 material and water in the leaves of Mesemhryanthemum does not vary 

 greatly with age. These and probably all succulent forms are char- 

 acterized by an exaggerated production of mucilages or pentosans, 

 and have certain impUed cycles of metabolism, including an incomplete 



* Donaldson, H. The relation of myelin to the loss of water in the mammalian nervous system 

 with advancing age. Proc. Nat. Acad. Sc, 2: 350. 1916. Hatai, S. Changes in the composi- 

 tion of the entire body of the albino rat during the life span. Amer. Jour. Anat., 1 : 23. 1917. 



* Ostwald, W. Ueber zeitlichen Eigenschaften der Entwickelungsvorgange, p. 49. 1908. 



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