140 
Journal of Agricultural Research 
V6I. XXIV. No. 2 
STABIUTY OF SAP DENSITIES 
It has already been indicated that the sap density of a given species 
is by no means constant. In order that progress may be made in the 
use of the sap density measure, or the osmotic pressure determination, 
as an indicator of the relations between plant and environment, it is 
extremely important to realize (i) that the tree may pass annually 
through a definite cycle of changes and (2) that the current sap density 
may be quite largely influenced by current atmospheric conditions as 
well as water supply. If the tree were always able to supply as much 
water , as was demanded by the losses at its leaves, then only a cyclic 
change would be apparent, dependent on cell division and photo- 
s)mthesis, or primarily on temperature and light conditions. It might 
be said, therrfore, that the species which shows the least fluctuation 
with current atmospheric conditions is the one best able to supply any 
demand for water, and it would seem that this species would best endure 
a long drought in the soil. 
. As to the cyclic change in sap density, it would seem that the following 
describe the general conditions of the seasons which bring it about: 
1. In the spring we have rising temperatures and increasing duration 
mid intensity of sunlight, which alone would increase the osmotic pressiue 
in favor of the plant as against the soil. Coupled with this there is 
usually an abundant supply of moisture in the soil immediately after 
its thawing, often augmented by the melting of large masses of snow 
and by heavy rains. These conditions cause a heavy inrush of water 
and, because the atmospheric demands are at this season very moder¬ 
ate, ato extreme turgescence of the tissues already formed becomes not 
only possible but unavoidable. It is believed this turgescence is the 
foundation for cell expansion and division, from which the new tissues 
arise. 
2. With the advance of spring and advent of summer the moisture 
sujiply of the soil is usually much reduced, but even if this were hot the 
caiSe the fannation of new tissues might be checked because of the very 
great increase in transpiration, due both to greater atmospheric demands 
and water losses from the new succulent tissues already formed. In 
the conifers, outward evidences of growth usually disappear abruptly 
early in the suinmer. The period of warmest weather, then, is not one 
for the formation of new tissues, but for the creation of the materials 
with which those already formed may be built up and solidihed. Fruit- 
of cporse, also dem^ds some of these materials. The sap density 
^<;mldi therefore, increase from the moment that cell divi^pn becomes 
ne^gil^e. 
3. The autunm season in temperate climates is almost universally the 
le^t favorable as regards current precipitation, and at this season the 
spil w^^t^ is most likely to have been completely exhausted. For this 
re^n tha amount that can be supplied is often inadequate for all the 
^ansphing mex^ and we witness the drying and falling of all decidu¬ 
ous leaver and of the pj^dest ones on the evergreens. U^ess the water 
snpply m the leaves becomes so low as to prevent the chemical processes, 
photpsynd^esis in the eyer^eens is not checked, and sap densities may be 
p^pected to teacdi their hfghest points, partly because the actual amount 
of water present is less than at any other season. 
4. While under some circumstances the winter season may be one of 
almost complete dormancy,5with photosynthesis stopped by low temper- 
