1907] HIBBARD—FORMATION OF MECHANICAL TISSUE 371 
4B, 5B, and 7B are within the limit of error. The remaining seven 
of the original seventeen were estimated with the aid of the micro- 
scope. These in nowise alter the general conclusion. 
Vinca major (greenhouse cultures) 
Twenty-three thrifty young stems of Vinca major, some of which 
were still growing upright, and others that had already bent over and 
Were growing in the usual drooping way, were subjected to tension by 
means of weights. The young upright stems were placed under ten- 
sion in the same manner as explained above for Helianthus annuus. 
The pendent stems were placed under tension by hanging the weights 
upon them without the use of the pulley. Cotton flannel was wound 
around the stem just above a node. Two loops of twine were then 
noosed around the stem over the cloth, and drawn tight. From the 
end of each noose hung equal weights, and thus the stem received the 
strain along its longitudinal axis. 
The duration of the experiments on the upright stems varied from 
two to four weeks, and the final weights varied from 1250 to 15008". 
The duration of the experiments on the pendent stems varied from 
eleven to twenty-eight days, and the final weights varied from 1000 
to 15008". Normally growing stems for comparison with tension 
stems were selected with regard to similarity in diameter, length, 
manner of growth, and vigor. In most cases sections were taken in 
norma] and experimental plants at equal distances from the stem apex. 
Observations with a magnification of 100 diameters show that 
in the majority of plants the whole xylem ring is thicker and com- 
posed of thicker-walled cells in the tension plant. The bast is more 
abundant in the tension plant. Under a higher magnification of 150 
diameters, it is noticed that the walls of the bast fibers are thicker in 
the tension plant. 
In all of the twenty-three plant stems under tension, five only do 
not show an increase in the absolute size of the xylem or in the thick- 
ness of the cell walls of bast or xylem. The following tables give the 
results obtained for a few of the representative plants and their con- 
trols. The preponderance of evidence, judging from the number of 
plants experimented on, shows that an increase in the quantity of 
xylem amounts to at least so per cent. In the table, T indicates 
