OSMOTIC PRESSURES IN THE POTATO PLANT 
201 
intact sieve-tube system, owing to the partial depletion of the tubes at 
certain points, are at once equalized by a more or less rapid displacement of 
the liquid contents in the corresponding direction. Whether the hydro- 
static pressure in the sieve-tubes owes its origin to the osmotic properties 
of the liquid contents or whether it is due to compression of the sieve-tubes 
by the highly turgescent adjoining tissues (leptome-parenchyma and com- 
panion cells) is still uncertain. Most probably both factors have a share 
in producing the pressure observed." He concludes: ''This matter evi- 
dently requires further investigation." 
The movement of food materials to the growing leaves of the young 
plant can be explained on the basis of high osmotic pressures in these organs, 
but the growth of the potato berries, of the young tomatoes, and of the 
potato tubers cannot be accounted for in any such manner since the osmotic 
pressure in all of these organs is the least of any in the plant. We must fall 
back upon the pumping action of the sieve-tubes as suggested by Haber- 
landt. 
One question more remains to be answered : Why does the potato plant 
suffer from tip-burn while the tomato is usually exempt? This may be due 
to structural differences in the leaves or to the resistance of the cells to 
incipient wilting, but it seems to be better explained by the continued 
movement of the elaborated carbohydrates upward in the tomato plant 
to form new foHage while in the potato they travel downward to form starch 
in the tubers. The tomato leaves may be better nourished from this food 
stream, and the continued formation of vigorous new leaves helps, too, to 
shade the older ones that are more susceptible to abnormal evaporation. 
Summary 
1. The potato plant early in the season records the highest osmotic 
pressure in the sap from the young stalks and leaves. 
2. During the very hot weather of July and August, the sap of the stalks 
develops a higher osmotic pressure than that from the younger portions of 
the plant. 
3. The high pressure in the stalks is due to the presence in them of 
sugars, especially of cane sugar. 
4. In September, after growth has been resumed, the young leaves again 
have the highest osmotic pressure of any portion of the plant. 
5. The osmotic pressure of the sap of the growing tubers is always low 
and is intermediate between that of the sap of the stalk and the sap of the 
roots, which latter is the lowest of all. 
6. The osmotic pressure in the older plants is higher than that in the 
younger ones and is due to the larger amounts of inorganic salts in the 
former. In very old plants, however, the soluble materials are removed 
to a considerable extent, and the osmotic pressure of the sap drops as a 
consequence. 
Vermont Agricultural Experiment Station. 
