10 2 BOTANICAL GAZETTE [august 



nation; while when the. tips were in the air the broad leaves 

 were produced whether the rest of the plant was in nutrient solu- 

 tion or distilled water. Fig. 2 shows a plant grown in nutrient 

 solution, andy?^. 8 shows similar plants in water. 



This seems to throw out the general conditions of nutrition, 

 so far as the salts are concerned, as having anything at all to do 



■ 



with the production of the air or the water form. The size of 

 the leaf primordia that c^n be changed from one form to the 

 other may be mentioned here. The oldest leaf primordia that 

 are still folded in the bud averasie about 6"""^ in leno^th. There 



j^V. K^^^^^ ^ .W -V.X.j^ 



are usually about nine primordia, and they successively decrease 

 in size, the smallest one that can be readily isolated being 0.2"^^ 

 long. On the creeping shoots there are about eighteen primor- 

 dia, the smallest being 0.06'"°' in length. A complete change 

 from the one form of leaf to the other often occurs within four 

 internodes, which means that a primordium may be turned into 

 either an air or a water leaf after it has attained a length of from 

 3 to 4^"^. 



5. Salts, ions, etc., as a stimulating cause. — The presence 

 in the water in solution of any such substance as salts, ions, 

 organic acids, or the like, as a stimulating cause to the water 



form is conceivable. Its possibility, however, is precluded by 

 the fact that the behavior is the same in redistilled water, and to 

 a certain extent in a saturated atmosphere. 



6. Relations to COg and to Og. — The gaseous content of the 

 water differs from that of the air. The air contains, per 1,000^*^, 

 209" O and 0.04^^ CO2 ; while water at 20° C. contains, per 

 1,000"^ 5.7^*= O and 0.3^^ COg, about 3V the amount of O, and 75 

 times the amount of CO^. But the rate of diffusion in the water 

 is much slower than in the air, and the cell may be able to use 

 both gases much faster than the slower diffusion will supply 

 them. The leaves cannot be lacking in oxygen during daylight, 

 for the evolution of bubbles of this gas indicates the excess of 

 the amount generated over that used. But the gas will tend to 

 pass off from all surfaces of the cell equally, and as much per 

 unit of surface will pass into the large and numerous air cham- 



