132 AMERICAN MIDLAND NATURALIST. 
THE STEM. 
The hypocotyl, the first internodes of the seedling, the branches 
of the mature tree, and finally the peduncle of the female inflo- 
rescence, these represent portions of the stem of which the structure 
must be considered in order to obtain a complete view of the internal 
morphology of this organ. To ascertain the primary structure of 
the stele the hypocotyl is especially instructive, while the older 
branches of the tree show us the tissues in their final shape with 
the secondary formations. In regard to the origin of the secondary 
mestome-strands, we remember that these most frequently develop 
from interfascicular cambium, or from a procambium *; however, 
several and very distinct modifications are known, more so from 
herbaceous than from woody stems, and some of these structures 
may be quite complicated. As regards the peripheral tissues these 
are, also, more or less involved and subject to change, when in- 
crease in thickness takes place, hence a comparison between young 
and mature shoots becomes necessary. In beginning with the 
hypocotyl of the seedling ( Fig. 1), we notice at once that all the 
peripheral tissues are preserved although the stele shows already a 
marked increase so as to obscure its primary structure. Viewed in 
cross sections the stele represents a solid circular zone of leptome, 
cambium and hadrome enclosing a homogeneous, thinwalled pith, 
and this structure exists from the subterranean base to the apex 
beneath the cotyledons. In the basal portion it is very difficult to 
locate the primary mestome-strands, while toward the apex their 
position becomes readily noticeable by the presence of stereomatic 
strands in the pericycle, and by the primary vessels being more dis- 
tinct, relatively narrow and of a darker color than the others. 
There are, thus, in all eight primary mestome-strands in the hypo- 
cotyl which traverse this parallel with each other and with the sur- 
face. But how these primary strands become connected with each 
other must be studied from a still younger stage than the one figur- 
ed (Fig. 1.) We must examine the seedling before the cotyledons 
have ceased to grow, and before the plumule has developed into a 
shoot of several internodes. At this stage the stele shows very 
plainly eight primary collateral mestome-strands separated from 
each other by strata of thinwalled parenchyma (Fig. 8.); or at a 
* Sachs: Lehrbuch der Bo:anik. Leipzig 1874 p. 113. Haberlandt: 
Physiol. Pflanzenanatomie. Leipzig 1896, p. 491. 
