. Senescence and rejuvenescence. Age; Reproduction. u. Figs, i, 2.—Planaria dorotocephala: Fig. i, a well-fed animal 2S mm. in length; Fig. 2, an animal reduced by starvation from 25 to 4 mm. SENESCENCE AND REJUVENESCENCE Logically our definition of growth might well include both positive and negative growth, or production and reduction, but since the word growth has come to be so generally associated with an increase in substance it is perhaps inadvisable to attempt to change its meaning. We may then retain the word growth for posi- tive growth or production, and use the term reduction for ne
. Senescence and rejuvenescence. Age; Reproduction. u. Figs, i, 2.—Planaria dorotocephala: Fig. i, a well-fed animal 2S mm. in length; Fig. 2, an animal reduced by starvation from 25 to 4 mm. SENESCENCE AND REJUVENESCENCE Logically our definition of growth might well include both positive and negative growth, or production and reduction, but since the word growth has come to be so generally associated with an increase in substance it is perhaps inadvisable to attempt to change its meaning. We may then retain the word growth for posi- tive growth or production, and use the term reduction for negative growth. But in so doing we must not forget that both these processes are in the broad sense, though not necessarily in the chemical sense, reversible, and that any adequate conception of the relation between the substratum and the dynamic processes ia the organism must be based, not on growth alone, but upon both growth and reduction. In other words, the activity of the organism may either increase or decrease the amount of its substance according to conditions. The question has often been raised whether the increase in the water-content of the organism is to be regarded as growth, or only the iacrease in the structural substance. Some definitions of growth have taken the one view, some the other, but if water is included among the sub- stances concerned in growth we have then to determine whether increase in water- content is in all cases to be regarded as growth, or whether we shall make a dis- tinction between growth and passive dis- tension due to external factors. Here again views differ. As a matter of fact, various investigators have shown that the imbibition of water is a very characteristic feature during at least certain stages of what we are accustomed to call growth: on the other hand, loss of water is a. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of
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