(426 ) 
tropa. This is connected with the fact that the roots of the 
first two increase in size to two or three times their original 
diameter, while the roots of the last two undergo but little 
alteration in dimension. The growth of Ayfopitys and 
Monotropa is due to the activity of the cortex. In Pleros- 
pora and Sarcodes the cortex is active, and after exfoliation 
the sub-epidermal! cells undergo division in the planes of the 
radii, and growth takes place at other places in the cortex. 
The central cylinder of Sarcodes and Pterosfora is least 
reduced and its development may be traced in them with some 
certainty. Here the five or six xylem bundles alternate with the 
simple phloem enclosing a well marked medulla. The first 
step in the secondary growthis the sclerotization of the medulla, 
and is followed by the lignification of this tissue in Sar- 
codes. Nextthe phloem gives rise to a cambium which de- 
velops wood internally, and bast on the outside. The latter 
consists for the greater part of elongated elements of narrow 
lumen which do not undergo any marked thickening of the 
walls. The wood formed by the cambium joins directly on 
to the lignified medulla. The inner ends of the primary 
medullary rays also undergoes sclerotization to some extent, 
but the outer portions show as broad bands one or two layers 
in thickness with the characteristic appearance of being com- 
pressed tangentially. The advance of the cambium is at 
first fairly regular, as the cambium zone moves outwardly 
beyond the first ring formed the transformation into vessels 
is accomplished with such disturbance or variance from the 
customary manner that it is not possible to draw a line separ- 
ating the two regions. Furthermore some of the cambium 
cells of great size remain as great thin-walled elements in the 
wood, or these may be arranged in radial lines simulating 
tertiary rays (Plate 12, Fig. 11). A region of cambiform 
elements four to six layers in thickness may be seen entirely 
surrounding the xylem. The structure formed by this be- 
havior of the cambium resembles that of a stem, and indi- 
cates that the roots of the two genera in question may attain 
an age of two years or more. 
