So aa 
1909] HIBBARD—FORMATION OF MECHANICAL TISSUE 377 
were chosen from the same head. No further selection was made. 
The Ricinus seeds were all taken from one variety. All the seedlings 
used for these experiments, except where otherwise indicated, were 
cultivated in an open field during the summer of 1905. The ground 
was carefully prepared with a mixture of clay, swamp muck, and 
manure. This was thoroughly spaded, and afforded a rich, firm soil, 
from which the seedlings were not easily uprooted when under tension. 
Two series of experiments were run through, one in the earlier 
and the other in the later part of the summer. The duration of the 
experiments was generally two weeks, after which time the plants 
were dug up, together with their controls, and their root systems 
carefully washed and placed in 50 per cent. alcohol. The root sys- 
tems of control and tension plants were first compared with reference 
to their external form and size and later with reference to their internal 
structures. The control plants were chosen from the same locality 
in which the plants under experimentation were grown and were 
selected with reference to similarity in size and vigor. 
I. EXTERNAL ROOT FORM 
To determine the influence of tension on the root system, a care- 
ful and close comparison was made between the control and tension 
plants. Special attention was given to observations upon the main 
and secondary roots. At times it was noticed that the lateral roots 
attained a greater development on account of the small or aborted 
growth of the main root. On the other hand, the main root would 
sometimes reach quite an enormous growth, in consequence of which 
the lateral roots would not attain the usual size. This great growth 
of the main root occurred far less frequently than that of the lateral 
roots. Thus what may be called a compensatory regulation holds 
true with reference to these two orders of roots. This compensatory 
regulation, then, must not be confused with the evident strengthening 
and increased development of roots due to tension. In our compari- 
sons of the normal and tension plants this fact had’always to be kept 
in mind. 
The number, average length, and generally vigorous condition of 
the roots and rootlets were determined. - It was found that in general 
the main root of the tension plant was straighter, tapered more 
