200 
Walter Stiles 
Further observations of Ursprung and Blum show that after rain 
the suction pressure is considerably reduced, as, for instance, in one 
observed case of palisade cells, from about 15-6 to ii-i atmospheres. 
Also, as with osmotic concentration, there appears to be a daily 
periodicity in the values of the full suction pressure, there being in 
general a minimum in the early morning and a maximum in the 
afternoon. 
In later papers Ursprung and Blum (1918 a, 1918 b, 1919) have 
extended their observations to other plants, and have in general 
confirmed the results of their earlier work. Thus in ivy they 
found that the suction pressure increased with distance from the 
absorbing zone of the root in any particular tissue, while in a cross- 
section through an organ the suction pressure increased with dis¬ 
tance from the water conducting tissue. The only exception to this 
rule was found in the absorbing zone of the root, in which, as might 
be expected, the suction pressure is greater in the cortex than in the 
piliferous layer. 
In their most recent paper (1921) these same authors pay parti¬ 
cular attention to the absorbing zone of the root in Phaseolus vulgaris 
and Vida Faba. They find that the suction pressure increases from 
the piliferous layer inwards as far as the innermost layer of the cortex 
outside the endodermis, but that the suction pressure of the latter is 
considerably less than that of the cortical cells outside it, and higher 
than that of the pericycle within. They regard this anomaly as most 
probably explained by the presence of different suction pressures at 
different points on the surfaces of the cells of the endodermis (cf. the 
last section of this chapter). The suction pressure as measured is the 
mean suction pressure in cases where the suction pressure varies over 
the surface of the cell, and if the suction pressure on the outer side of 
the endodermal cells should be higher than that of the neighbouring 
cortical cells, and on the inner side lower than that of the adjoining 
pericycle cells, the entry of water into the central cylinder can be 
explained. 
As regards the value of the suction pressure in other cases, Stiles 
and Jprgensen (1917 b) have shown that a sample of potato tuber 
neither gained nor lost weight in a M /4 solution of sucrose, which 
indicates a full suction pressure of about 6-5 atmospheres. An experi¬ 
ment with carrot root indicated a suction pressure of about 17 atmo¬ 
spheres. The suction pressure in these cases depends obviously on 
the previous history of the experimental material, and especially on 
the degree of desiccation. 
