1912] BRUSH—MECHANICAL TISSUE 469 
2. When contact is increased by pressure, a further increase 
_ in the strength of the tendril is produced. 
3. When the factor of tension is added to that of contact, a 
still greater strength results to the tendril. 
ANATOMICAL STUDY 
1. General anatomy of the Passiflora tendril——A cross-section 
of a tendril of Passiflora caerulea, in accordance with the observa- 
tions of MAcDovucat (13) and Worcitzky (26), reveals the follow- 
ing tissues, beginning at the outside: epidermis, collenchyma, 
thin-walled parenchyma, bast, xylem (which forms a complete 
ring, due to secondary growth), and in the center pith. In mature 
tendrils the pith entirely fills the central part except in the basal 
portion, where there is a central cavity within the pith. The 
walls of the xylem and bast are much thickened, and so are (as 
Worcirzky has noted, p. 34) the walls of the pith. The xylem 
becomes lignified, also the bast somewhat, and, as MacDouGAL 
observed, lignification extends to the pith also. 
At the base the arrangement of tissues is very nearly radial, 
but in the portion in contact a marked dorsiventrality is seen, 
which is due principally to the development of the xylem to a 
much greater extent on the side in contact. A section midway 
between the apex and base of the tendril shows a slight dorsiven- 
trality, a somewhat greater amount of xylem being formed on 
the concave side. 
2. Study of sections; experiments on entire tendril (free, with 
contact alone, and with contact and tension).—Sections were made 
through the middle of the tendrils, as this was found to be the 
place at which the break invariably occurred in these experiments. 
Tendrils as near as possible to the average breaking strength were 
taken for sectioning. A comparison of sections reveals the fol- 
owing. 
The mechanical tissue of the free tendril is limited to a small 
area of xylem on the concave side, and only the four primary 
bundles on the opposite side. The xylem cells are quite thin- 
walled compared with the xylem of the other tendrils in this 
experiment, and the primary bundles of the opposite side are 
