360 MacDOUGAL— HYDRATION AND GROWTH. 



of woody stems, thin leaves and 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 disminishes 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 earlier 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 191 8. 



Etiolated plants furnish examples of growth with a small increase 

 in proportionate dry weight, but chief interest attaches to plants 

 which normally show such action, and the most striking illustrations 

 are to be furnished by the organs of succulent plants and by fruits. 

 The relative amounts 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 fibrovas- 

 cular tissue, may cause an added amount to be formed. The propor- 

 tion of dried material and water in the leaves of Mesembryantlicmiim 

 does not vary greatly with age. These and probably all succulent 

 forms are characterized by an exaggerated production of mucilages 

 or pentosans, and have certain implied cycles of metabolism includ- 



1 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 Composition of tlie Entire Body of the Albino 

 Rat during the Life Span," Amcr. Jour, of Anat., i : 23. 1917. 



- Ostwald, W., " Ueber die zeitlichen Eigenschaften der Entwickelungs- 

 vorgange," p. 49. 1908. 



