Transactions Texas Academy of Science, 
91 
them in such manner as to produce a strain on the tissues of the outer 
curve. On the klinostat, gravitation no longer acts continuously upon 
the tissues of one side of the stem, but, by means of the rotation, its 
effects are equally distributed on all sides. Evidently if proper care be 
taken, the effects of one sided light supply may also be obviated either 
by working in diffuse light or in the dark room where all light is ex- 
cluded. Elfving concluded that as tension was the only known agency 
at work, it must be ( the cause of the thickenings which occur under these 
conditions, both in light and in the dark room. 
Elfving’s experiments were carefully repeated and his results in every 
case sustained. However, though tension is the only known factor at 
work, it is not safe to ascribe definitely these results to its action, since 
other and unseen forces may also take a part. 
II. In order to exclude tension, but still to allow full play to the 
action of gravitation, portions of the stems of the plants named were 
enclosed in plaster of Paris casts and the whole laid horizontal. The 
portion so enclosed can not curve under geotropic influences. This geo- 
tropic curving is immediately^ produced by the more rapid growth in 
length of the cells of the lower or earthwards side of the stem, while 
those of the upper grow normally. Hence there is a stress exerted upon 
the tissues of the latter side." Since all growth in length is inhibited by 
the cast, there can obviously be no stress due to the cause named. The 
portion of the stem behind the cast was prevented from curving by the 
weight of the gypsum, while that part in front could curve freely. Sec- 
tions taken from all parts of the stem showed a general thickening of 
cell walls of the upper side, though this was not so marked in the parts 
enclosed in the plaster, as was to be expected, since growth was necessarily 
restricted there and the stem suffered more or less etiolation. 
III. Young stems of the bean were now forcibly bent in the lower 
portion and fastened by threads in a nearly horizontal position. After 
geotropic curving of the free part had taken place, the plants presented 
an b figure in which the curve A was due to the mechanical 
stress applied, and that of B to normal geotropic curving. At A the 
strain upon the tissues must have been considerable. After about ten 
days sections were cut and showed thickened walls of the bast, collen- 
chyma and epidermis of the upper side. In the bast, however, the num- 
ber of cells whose walls underwent thickening was always less than in 
the normal plant. This fact was observed in all cases of thickening and 
is especially noticeable in Ricinus. 
IY. When the plant has reached an age such that the stems are no 
longer pliable and do not curve under geo tropic influences, no mechanical 
means are required to hold them in a horizontal position. Here alsc 
