42 
TENTH REPORT. 
PROBLEM OF THE CAUSES OF THE FORMATION OF MECHAN- 
ICAL TISSUE IN PLANTS. 
It is quite generally believed by botanists that the woody, supporting 
tissue which a plant forms comes into being largely through hereditary 
influence. Considerable study has been made of the possible influence 
of external factors also. It has been found that an increase of transpira- 
tion of water increases the mechanical tissues; that in some plants, leaf- 
development and shoot-development cause an increase of mechanical tissue, 
even before leaf and shoot are mature enough to become functional. The 
various tensions which a tree trunk and root must undergo in swaying in 
the wind, in supporting their own weight, in supporting the weight of 
fruit, etc., are popularly supposed to induce the formation of support- 
ing tissue. 
Seventeen years ago, a number of experiments were made which seemed 
to show that longitudinal traction of stems of seedlings and stems of leaves 
brought a quick development of mechanical tissue. A repetition of those 
experiments more recently seemed to show that traction induced generally 
no increase of strength in the plant members. 
This was the condition of the subject when the question was taken up 
several years ago in the botanical department of this University for careful 
testing. The reports by those who follow me on the program will show 
that longitudinal traction of stems, roots, and tendrils brings an increase 
of mechanical tissue, though not a remarkably large increase, and that 
swaying in the wind produces a large increase of mechanical tissue in stems. 
Frederick C. Newcombe. 
