1907] HIBBARD—FORMATION OF MECHANICAL TISSUE 369 
The remaining five plants of the ten used in the above experiment 
were allowed to grow with the constant pull of weights until they had 
produced flowers. The purpose of this experiment was to note the 
influence of tension continued for a long period. These plants 
because of the winter season were rather weak. Cross-sections of 
these stems at 7°™ above the ground show a closed xylem ring. The 
area and weight methods as described above were employed for detect- 
ing differences. Even in these examples we find no self-regulatory 
development of mechanical tissue in response to tension, nor are the 
results on old stems any different from those on young ones. The 
final weight carried by each plant was 23008". 
The above experiments on Helianthus annuus were repeated with 
twenty-five other individuals and the same methods for detecting 
differences were used. Only a few of these plants were allowed to 
mature, as young plants give just as reliable results and are much more 
easily handled. The duration of the experiments varied from two to 
four weeks. The final weights varied from 600% to 2860". The 
general result was that tension, gradually increased and in the direction 
of the longitudinal axis, did not induce a self-regulatory development 
of mechanical tissue in the sunflower stem. In this we agree fully 
with the results of BALL. 
Helianthus annuus (field cultures) 
Seventeen sunflower plants were subjected to the same condition 
of strain, but to other external conditions which were not so uniform 
as those in the greenhouse. These experiments were conducted in 
the open field during the months of July and August 1905. 
An examination of the cross-sections of these stems, taken at 1°™ 
above the first lateral root, gives the general result that was found 
to be true in the case of the plants grown in the greenhouse, namely, 
that an increasing tension along the longitudinal axis does not call 
forth a self-regulatory development of mechanical tissue in the sun- 
flower stem. By the area method only three tension plants show an 
increase of mechanical tissue over that of the controls. Two control 
plants show an increase in mechanical tissue over that of the tension. 
The others show slight differences that fall within the limit of error. 
Ten examples from the seventeen are given in Table I. 
