May 3i, 1924 Cell Sap Density and Environmental Conditions 
897 
leaf material for the sap density determinations. Plants on exposed sites are 
usually thicker, more hairy, have a more highly cutinized and thicker epidermal 
wall, a more compact palisade parenchyma, and more closely crowded stomata 
than on less severe sites. 
The powers of accommodation or adaptation exhibited by plants are evident 
in the density of the sap. The extent of adaptation varies with the plasticity of 
the species. The curl-leaf mahogany ( Cercocarpus ledifolius) , manzanita ( Arcto - 
staphylos pungens) and mountain myrtle ( Pachystima myrsinites), which have 
thick leathery leaves, and Ceanothus velutinus , with a very sticky leaf, are species 
possessing variable sap concentrations. On the other hand, the succulent Opuntia 
and Sedum have surprisingly low sap densities. Those plants which possess a 
low transpiration rate as a result of anatomical modifications, the presence of 
pubescence or pulverulence, and those plants having pronounced water storage 
tissue in succulent leaves or stems are capable of maintaining surprisingly low 
concentrations of the cell sap. 
In many plants the major physiological activities, such as photosynthesis, 
absorption, and transpiration, arc materially modified by special protective 
coverings and by anatomical and even functional adaptations. The rate of 
transpiration, for example, may be materially reduced by structural modifica¬ 
tions. Since osmotic phenomena are so intimately connected with all these vital 
physiological functions it is clear that an intimate relation also exists between 
structural adaptations and the density of the cell sap. 
Our knowledge of the water relations of some western mistletoes also has been 
augmented by a study of the osmotic concentration of the sap of these parasites 
in comparison with that of their hosts. Determinations on the sap of Razoumof- 
skya douglasii on Douglas fir in Big Cottonwood Canyon and R. americana on 
lodgepole pine on Beaver Creek, both made in September, 1920, and R. cryptopoda 
on western yellow pine and Phoradendron juniperium on one-seed juniper ( Juni - 
perns monosperma), in August, 1921, at the Fort Valley Forest Experiment 
Station near Flagstaff, Arizona, are given in Table XIX. 
Table XIX.— Sap densities of some western mistletoes and their hosts 
Species 
Depression 
of freezing 
point. 
Osmotic 
pressure. 
Jiazoumofskya douglasii ......__ 
Degrees C. 
1.64 
Atmos¬ 
pheres 
19.7 
On Pseudotsuga taxifolia .....„... 
0.98 
11.8 
Jiazoumofskya americana. ________ 
1.62 
18.3 
On Pinus contorta ....... 
1.41 
17.0 
Razoumofskya cryptopoda ..... 
1.46 
17.6 
On Pinus ponderosa scopulorwi - ~ ..... 
1.42 
17.1 
Phoradendron juniperium .___... 
1.30 
15.6 
On Juniperus monosperma ....... 
1.28 
15.4 
These results indicate that the osmotic density of the sap extracted from the 
tissues of the parasite is generally greater than that from the mature leaves of 
the host. 27 A higher osmotic concentration of the sap of the parasite is not a 
necessary condition for the temporary success of the parasite. The parasite 
should be able to draw from the relatively dilute solutions in the stem of its host 
in competition with organs of actually higher osmotic density, except at periods 
when the supply of available moisture is limited, just as young leaves are able to 
draw water in competition with old leaves of high osmotic density. The Arizona 
27 The studies of Harris (58) on desert Loranthaceae and those of Harris and Lawrence (49) on the para¬ 
sitic Loranthaccae of Jamaica accord with the above findings. 
