476 BOTANICAL GAZETTE [JUNE 
tions and experiments presented in this paper. This accords with 
the observations of HEGLER (8) that the retarding effect of tension 
is closely related to the daily periodicity of growth in length, 
which seems to show that tension acts as a true stimulus upon the 
plant cell. As to the exact method by which this increase and 
strengthening of tissue takes place we are unable to say, since we 
know very little, as yet, of the nature of the changes taking place 
in the cell and especially in the cell wall, under the influence of 
tension. 
‘It seems not unlikely that this increased growth is due to 
increased hydrostatic pressure in the cell, since HEGLER found a 
higher hydrostatic pressure in plants which had been under ten- 
sion than in plants growing normally (7, p. 416). 
The state of tension in which the cell wall might be, may act 
as an irritation-stimulus for the laying down of more tissues 
either by apposition or intussusception; here, however, we should 
have to assume the cell membrane (at least in part) to be composed 
of living protoplasm, for which assumption we have no well founded 
evidence (see PFEFFER 18 [Ewart transl.], 1:485). 
_ As to why this thickening in Passiflora did not occur in the 
xylem also under the influence of tension, we are unable to say; 
this difference in response is probably due to fundamental differences 
in these tissues. 
This investigation was conducted at the University of Michigan 
under the direction of Professor F. C. NEwcomBE, to whom I 
wish to express my sincere thanks for his kindly interest and helpful 
suggestions. 
Wasuincton, D.C. 
LITERATURE CITED 
1. Batt, O. M., Der Einfluss von Zug auf die Ausbildung von Festigungs- 
gewebe. Jahrb. Wiss. Bot. 39: 305. 1904 
. BARANETSKY, J., Die tagliche Periodizitaét im Langenwachstum der Stengel. 
Mém. Acad. St. Pétersbourg 27: no. 2. 1879. 
. Borpner, J. S., The influence of traction on the formation of mechanical 
tissue in stems. Bor. GAz. 48:251. 1900. 
4. CHAMBERLAIN, C. J., Methods in plant histology. Chicago. rgor. 
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