1907] HIBBARD—FORMATION OF MECHANICAL TISSUE 365 
exerted an influence on the lignification of the cell walls of many of 
the tissues in the stalk. In the apple and the pear there was an 
abundance of well-lignified sclerenchyma and hard bast, which 
occurred in the vegetative shoot only sparingly, if at all. 
BooDLE (:02) states that the walls of the sieve tubes and com- 
panion cells in Helianthus annuus become lignified as a result of 
strain. We also read “that the slight lignification of the parenchyma- 
tous parts of the pericycle and medullary rays unites the primary 
sclerenchyma strands into a more definite mechanical system attached 
to the strong xylem by the medullary rays.’”’ Finally, he says, “this 
must give greater rigidity, which no doubt is required by the heavy 
fruiting capitula borne by the plant.” 
On the other hand, in opposition to this theory, KELLER (:04) 
finds that pull as such does not call forth a regulatory strengthening 
of mechanical tissue in fruit stalks. Fruit-bearing in itself does not 
cause a thickening of parts nor exert an influence on the lignification 
of the cell walls. Upon orthotropic flower stalks, a strong or light 
pull in the direction of the long axis exerts no influence on the develop- 
ment of mechanical tissue. Displaced stalks under tension show 
no self-regulatory thickening, but certain anatomical changes do 
take place. These changes are not due to tension, but simply to the 
alteration in the position from orthotropic to plagiotropic, and 
in turn are directly referred to differences in the degree of strain 
between the upper and lower sides. These conclusions do not fall 
in line withthe view that has held ground for some time, namely, 
that the mechanical development of the stalk goes hand in hand 
with the development of the fruit. Correlative growth, it is said, 
is no explanation for this phenomenon. This interpretation is yet to 
be justified. If this be true, our previous ideas must undergo trans- 
formation. 
As a conclusion of the historical part of this paper I might sum- 
marize these views: Tension has no influence on the increase of 
mechanical tissue in any stems examined, in any petioles, nor in 
branches, except in Corylus avellana. It has some effect on 
twining petioles in that there occurs a thickening and lignification of 
certain tissues. The effect on tendrils has not yet been accurately 
determined. Until further data are gathered to show the contrary, 
