604 



SCIENCE 



[N. S. Vol. XLIX. No. 1278 



pounds in fact yield no energy and furnish no 

 building material. They may act as catalyzers 

 or as releasing agents, and as controls of water 

 absorption or as guides in colloidal arrange- 

 ment, but they are not " food-material " in any 

 sense. The constituents of fertilizers should 

 be designated as "culture salts" and as such 

 have all of the importance which has been im- 

 puted to them.; a determination of the compo- 

 sition and proportion of salts in a culture so- 

 lution whidh wiE induce maximum produc- 

 tion of grain, fruit or forage is a problem of 

 the first rank now happily receiving something 

 like an adequate investigation. 



The foregoing suffices to account for the me- 

 chanics of growth or expansion of a single- 

 celled or naked organism. The development 

 of complex, massive or higher organisms espe- 

 cially in plants, however, is accompanied by 

 the formJation or deposition of an outer layer 

 of denser consistency which occurs at any 

 phase boundary of colloidal material. This 

 membrane so-called is in any case a product of 

 the surface energy of the mass or system of 

 living material in the cell and of the material 

 in contact and its constituition, and even its 

 structure must vary as widely as that of the 

 prottoplasm which produces it. 



External to the membrane is the cell-wall 

 which begins to he formed around plant cells 

 as soon as they divide or are separated and this 

 wall increases in rigidity and offers greater re- 

 sistance to stretching as it grows older. 



The arrangement in question, therefore, is 

 one in which the expanding and growing pro- 

 toplast is enclosed in a sac or bag of its own 

 making and which acts as a screen not only in 

 allowing some materials to pass while others 

 are shut out, but also is so constructed tiaat 

 some solutions pass through it more readily 

 into the cell than out of it, these being simply 

 examples of isome of the many facts discussed 

 under the designation of permeability. The 

 external screening membrane takes on a spe- 

 cial significance in connection with the os- 

 motic action of the vacuoles. 



These sacs were at one time thought to have 

 a morphological value, but it is now under- 

 stood that almost any hydrating colloidal mass 



may exhibit syneresis in which cavities or 

 canals are formed in which the colloidal mate- 

 rial accumulates in an attenuated or liquid 

 condition. These syncretic cavities increase 

 by absorption of water and by the time the 

 protoplasm of the cell has attained about half 

 of its ultimate bulk in some instances, these 

 cavities have enlarged to occupy a space as 

 large as the protoplasm and acting as vacuoles 

 by which they are ordinarily known, eventually 

 fill a much larger space. The expansion of 

 these vacuoles and the consequent increase in 

 voliune of the cell constitutes part of the en- 

 larging action of growth, and this expansion 

 takes place by the force of osmotic action, 

 and the result of such stretching is to set up 

 a tension ordinarily designated as turgidity. 

 The vacuoles continue to hold some of the 

 colloidal material and may also carry in solu- 

 tion almost any substance in the cell which 

 may be passed into them by osmosis or diffu- 

 sion, including sugars, salts, acids, amino- 

 compoimds, etc. 



The enlargement of the individual masses 

 of living cells in organisms entails a certain 

 amount of work which in the earlier stages is 

 derived almost entirely from imbibition or 

 adsorption, and while such action continues 

 throughout the growth or life of the living 

 matter, there is in addition the stretching ac- 

 tion exerted by the expanding vacuoles by os- 

 motic action. The growing regions or plants 

 at all times include cells in all of these stages, 

 from the newly separated protoplasm which is 

 exipanding entirely by imbibition of water and 

 incorporation of new material, others in which 

 the syneretically formed vacuoles are increas- 

 ing and thus adding to the volume of the cell 

 by osmotic action, and others approaching ma- 

 turity in which the vacuole may have attained 

 such size as to occupy many times the space 

 of the living matter which may indeed now be 

 but a sac with its layers of irregular thickness 

 lying internal to the wall, which now has be- 

 come dense and rigid. 



The measurement of the growth of a stem, 

 root or fruit of a plant will, therefore, show 

 the composite changes in volume of cell masses 



