1900] PERENNIAL HERBS 173 
result of the root contraction noticed. The radial longitudinal 
walls of their cells, being quite straight at the beginning, as con- 
traction sets in become marked with wavy foldings. In the 
endodermis the waves are strongest in that longitudinal band of 
the wall which corresponds to the well-known dark spot on the 
cross-section. 
~ As each root is fastened at its tip to the earth and at its base 
to the tuber, in consequence of the contraction a tension is set 
up init, and the root must give way at the point less firmly 
fixed. Asa matter of fact, mostly the base of the root moves 
towards the tip and pulls the tuber with it. Thus, by the com- 
bined action of all the roots, since all in their basal part point 
steeply downwards and differ but little in direction, the tuber is 
drawn down into the earth a certain amount every year. 
Thus the contractile roots determine to a great extent the 
position of the tuber. The latter is found sometimes upright, 
sometimes lying horizontally, not seldom even turned over with 
the bud directed downwards, so that the leaf-stalks and stems 
have to make a strong curvature in order to attain an upright 
position. The situations are largely due either to a uniform or 
one-sided pull of the roots. I am not able to say with certainty 
which direction of growth the tuber would take up by itself, and 
whether its behavior in this respect would be the same under 
different external conditions. The annual prolongation of the 
tuber is in larger individuals from 6 to 10™™ ( fig. 6), and if the 
— of growth were always upright, the dragging action of 
the —— Moud of course be of essential importance for keeping 
Sfowing point in place and preventing its emerging from 
the earth. 
. Soe ok nee are furnished with root halts; and form oo 
the tim: a . at the first order in their terminal pee ; 
their scars the withering of the leaves all the roots die ) mp 
this dikes at then found in a ring-like zone on the surface 
In the a . : — 
S of the scale leaves lateral buds originate, four oF 
fiy : 
© every year, which persist after the leaves which support 
