Transactions Texas Academy of Science. 
89 
1. What effect is produced by tension acting in the normal vertical 
direction of growth? 
2. What is this effect when applied in any other than the vertical ? 
The published results of Hegler have a twofold significance — a philo- 
sophical and a practical, the first of which has already been indicated. 
If a new force acting upon an organism, can within a short period of 
time, produce such marked changes in its tissues, and what is still more 
remarkable, in some cases call into existence tissues which are normally 
not present, or at least, normally remain undeveloped, then the accepted 
theory of evolution, according to which all existing organisms and their 
parts have reached their present form and acquired their several habits 
only through a long process of adaptation extending through a wide 
range of time, is at fault. 
On the other hand, such results would indicate the possibility of in- 
creasing the tensile strength of timbers to any desired extent by a simple 
process of applying suitable weights to a young and growing tree and 
increasing these as fast as the plant had adjusted its mechanical tissues 
to the new strains put upon them. 
Certain other questions as to the behavior of plants, or their parts, 
when in positions other than the normal, and either subjected to tension or 
not, at least so far as artificial mechanical strain is concerned, the parts 
played by gravitation and the effects of difference in light supply, must 
also be considered. For example > in the cone bearing trees, the woody 
tissues of the under side, in Dicotyledons, those of the upper, are hyper- 
trophied in the parts which stand in oblique or horizontal position. 6 Ad- 
ventitious buds appear on the upper and rootlets on the lower side of 
many plants or their parts when in a more or less horizontal position. 7 
These effects have been found to be due in some cases simply to geotropic 
causes, in others to the influence of one-sided light supply and are prob- 
ably often the resultant of these two. 
A young and growing plant when laid horizontal and unrestricted, 
curves upwards in order to regain the normal position. This reaction 
is due to gravitation and is called Geotropism. If this curving be me- 
chanically prevented, a marked thickening of the cell walls of the tissues 
of the upper side occurs. 8 It must be noted here that, under these con- 
ditions, the tissues of the upper side are subject to a certain degree 
6 Nordlinger Der Holzring als Grundlage des Baumkorpers, 1871, p. 24. Weisner, 
Ber. d. Deutsch. botan. Gesellsch., 1895, p. 481, and 1896, p. 181. 
7 Vochting, Organ-Bildung im Pflanzenreich, 1868, pp. 148, 164. Sachs, Arbeit, 
d. Botan. d. Inst, in Wiirsburg, 1880, Bd. 2, p. 474. Wiesner, 1. c. See Pfeffer, 
Pflanzen physiologie, 1901, pp. 107, 125 and the literature there cited. 
8 Wortman, Zur, Kenntniss d. Reisbewegung, Botan. Zeitung, 1887, p. 819. 
