os peep eee 
Knox: STEM OF IBERVILLEA SONORAE ool 
The stems of cucurbits are usually five-angled, and only in 
exceptional cases seven-angled or terete. A transverse section 
through a stem of this type, or five-angled stem, exhibits two cir- 
cles of bundles, five in each circle, the outer standing in the angles, 
the inner in the furrows. The bundles are normally all bicollateral, 
possessing an outer leptome, a hadrome region, and an inner lep- 
tome. Both inner and outer leptome contain very large and 
prominent sieve-tubes, as well as much leptome parenchyma. 
The common opinion seems to be that there is no inner cambium, 
though the outer cambium is a wide and active zone peripheral to 
each hadrome strand. The bundles are widely separated, and lie 
ina large-celled ground parenchyma. The pitted vessels of the 
hadrome are exceptionally large, and in older stems are braced by 
thyllae. A continuous ring of stereome borders the outer limit of 
the pericycle, and there is a row of starch- filled cells, accompanied 
by various arrangements of chlorophyl-free collenchyma just 
beneath the epidermis. There are frequently hairs or glands on 
the epidermis, and sieve-tubes scattered through the cortex and 
pericycle. Inner secretory passages are wanting, and deposits of 
calcium carbonate seem to occur only in the leaves. As the stems 
are mostly annual a periderm does not form, and for the same 
reason no true bark is found. Many species show a lysigenous 
Cavity in the pith making the stem hollow, and because its sec- 
ondary growth chokes this cavity and fills it up Bryonta dioica is 
mentioned ds peculiar, 
In describing /éervillea it will be taken first as a primary stem 
at the end of the first year, and later as a secondary stem, and a 
diagram of the transverse section of the young shoot cut 24 cm. 
from its tip where the stem was fresh and green is given in PLATE 
24, FIGURE I. This shows the general outline of the section to 
be irregular with a possible tendency toward the seven-angled 
type, though its external appearance is smooth, the number of the 
blunt « angles” varies, and as it grows older it is always terete. 
The bundles are arranged in two rings, thoge of the outer being 
smaller. The number of those in the inner ring is always five, 
that of the outer ring varies from five to sever or even nine. An 
attempt is at once suggested to place the stem in one of the classes 
made by Lotar (12), Petersen (14), or Tondera (31). Different 
