76 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
preponderance of the growth of the second period (7 days) is an index 
to the preponderating influence in growth of the imbibition of water. 
An analysis of the substance of the stem at different levels below 
the tip reveals the same thing — a sudden increase in the amount of 
water from 73% at the tip to 88% at the first internode (II), 
reaching a maximum at 93% in the second internode (III), then 
falling slightly (92.7) to the fifth internode (VI, Fig. 2). The 
Fig. 2. Curve showing the percentage of water in successive internodes of Hetero- 
cention, from the tip to the fifth. After Kraus, Festschr. naturf. Ges. Halle, 1879. 
experiments and observations upon which these conclusions rest 
thus agree in assigning the chief r6le to water in the growth of 
plants. 
With animals experiments on this point were lacking. It seemed to 
me, therefore, a worthy thing to determine whether animals followed 
the same law in respect to the preponderating importance of water in 
growth. Accordingly I set to work to determine the percentage of 
water in the body of developing tadpoles at different stages. Eggs 
and embryos were weighed at various ages after removing superficial 
water. Then they were kept in a desiccator from which air had 
been pumped and which contained a layer of sulphuric acid to 
absorb moisture. After repeated weighings a condition was found 
in which the drying mass lost no more water (constant weight). 
The total diminution in weight indicated the mass or volume of 
free water contained in the organism at the beginning of the experi¬ 
ment. Numerous weighings were made during two seasons upon 
