To be content to shew that a certain rate of growth occurs in a certain organism under certain conditions, or to speak of the phenomenon as a ãreactionã of the living organism to its environment or to certain stimuli, would be but an example of that ãlack of particularity74ã in regard to the actual mechanism of physical cause and effect with which we are apt in biology to be too easily satisfied. But in the case of rate of growth we pass somewhat {125} beyond these limitations; for the affinity with certain types of chemical reaction is plain, and has been recognised by a great number of physiologists.
A large part of the phenomenon of growth, both in animals and still more conspicuously in plants, is associated with ãturgor,ã that is to say, is dependent on osmotic conditions; in other words, the velocity of growth depends in great measure (as we have already seen, p. 113) on the amount of water taken up into the living cells, as well as on the actual amount of chemical metabolism performed by them75. Of the chemical phenomena which result in the actual increase of protoplasm we shall speak presently, but the rûÇle of water in growth deserves also a passing word, even in our morphological enquiry.
It has been shewn by Loeb that in Cerianthus or Tubularia, for instance, the cells in order to grow must be turgescent; and this turgescence is only possible so long as the salt water in which the cells lie does not overstep a certain limit of concentration. The limit, in the case of Tubularia, is passed when the salt amounts to about 5ôñ4 per cent. Sea-water contains some 3ôñ0 to 3ôñ5 p.c. of salts; but it is when the salinity falls much below this normal, to about 2ôñ2 p.c., that Tubularia exhibits its maximal turgescence, and maximal growth. A further dilution is said to act as a poison to the animal. Loeb has also shewn76 that in certain eggs (e.g. those of the little fish Fundulus) an