230 BOTANY 



Coelenterata or Echinodermafc$. When an ambulaeral foot of a star- 

 fish or a sea-urchin is lengthened from a millimetre to several centi- 

 metres by filling with water from the water-vascular system, the water 

 has in this instance the same biological significance as in the elongation 

 of the plant organs, except that in the latter case the process is not of 

 repeated occurrence. 



The great advantage resulting from this easy method of enlarge- 

 ment is apparent from a consideration of the importance of a large 

 external surface for the nutrition of a plant. Assimilation is just so 

 much the more productive, the larger the exposure of green surface, 

 and the more accessible it is to the surrounding carbonic acid. In 

 like manner, the superficial enlargement is exceedingly advantageous 

 as regards the absorption of nourishment from the soil. It is accord- 

 ingly of great economic value biologically that the growth through 

 elongation is accomplished chiefly by the absorption of water. 



The absorption of water by living cells does not take place with the same 

 rapidity and without interruption as in the case of porous bodies. Before the cells 

 can take up additional water they must enlarge by actual processes of growth. 

 The water, penetrating the young cells by imbibition or by the force of osmotic 

 pressure, is uniformly distributed through the protoplasm, which fills the cell ; 

 in case the protoplasm is already abundantly supplied with water, it is instead 

 accumulated in vacuoles (Fig. 50). As the vacuoles contain also organic and 

 inorganic matter in solution, they exert an attractive force and give rise to further 

 absorption of water. The sap of the vacuoles would, in turn, soon be diluted and 

 its attractive force diminished, were it not that the regulative activity of the proto- 

 plasm soon provides for a corresponding increase of the dissolved salts, so that the 

 concentration and attractive force of the sap are continually being restored or even 

 increased. The separate vacuoles thus enlarged ultimately flow together into one 

 large sap-cavity in the middle of the cell, while the protoplasm forms only a com- 

 paratively thin layer on the cell walls, which now exhibit considerable surface- 

 growth. 



During this increase in the volume of the cell, the protoplasm has 

 experienced but little augmentation of its substance, or other modifica- 

 tion. The enlargement of the cell has been almost wholly produced 

 by the increased volume of water in the sap cavity, which, to dis- 

 tinguish it from the "nutrient water," "imbibition water," and 

 " constitution water " of the plant, may be designated " inflation water." 



As is often observed with the occurrence of many vital phenomena, 

 the rate of distension of the walls with the inflation water is not 



uniform, but BEGINS SLOWLY, INCREASES TO A MAXIMUM RAPIDITY, 

 AND THEN GRADUALLY DIMINISHING ALTOGETHER CEASES. As all 



the cells of equal age in an organ go through this process of inflation 

 at the same time, the phenomena of increase and decrease in the rate 

 of growth are apparent in the growth of the organ, and give rise to 

 grand periods of growth. Minor periods, or fluctuations in the 

 rate of growth, occurring within the grand periods, are due to irregu- 



