CO OO OT OL OO 
* 
Knox: Stem or IBERVILLEA SONORAE 341 
shoots are sometimes four and a half centimeters broad one can 
get intermediate stages between green lianas and tubers. The age 
of the plants is difficult to estimate. The seedling photographed 
has been in the greenhouse four years. The shoot in FIGURE 9 is 
over two. years old. The size of the ducts must seemingly be 
taken as the criterion, and each two or possibly three large tracheae 
must constitute the growth of a season. There is usually associ- 
ated with the fall growth the formation of unlignified wood- 
parenchymaat the side of the hadrome, so that the region presents 
the jagged appearance noted in FIGURE 13. That the large tubers 
are fifty years old is doubtless a most conservative estimate. 
The pharmaceutical character of the stem is only known em- 
pirically to the Indians, who regard it as very poisonous, but more 
so than has been found to be the case by Miss Emerson and Mr. 
Welker. The stem shows quantites of starch at the end of the 
growing season, but the shoots die back so short a distance that 
it is extremely improbable that the nutritive substances are with- 
drawn into the tuber. In the desert, the drier condition may 
effect a change in its habit so that the shoots shrivel further down — 
toward its base. The mechanics of the stem after the breaking 
up of the stereome-ring are those of a true liana. The plant is 
‘Not only quick of development in.a short rainy season as well as 
resistant in a dry one, but it is able to twine about surrounding 
woody growth and to expose a relatively large leaf-surface above 
the sandy levels. It is perhaps to be noted that the leaves when 
older have a white spotted appearance, and would doubtless prove 
to be good material for the study of cystoliths. 
The differential characters of the stem may be summed up as 
ie 
. The shape is terete, with from ten to fourteen bundles. 
2. It possesses endocyclic as well as ectocyclic and commis- 
Sural sieve-tubes. 
3. It has an active inner cambium. 
4. The obliteration of the sieve-tubes changes them into a 
secretory system of which the contents serve as wound-gum. 
5. There is a periderm with phellem and phellogen. 
6. Deposition of calcium carbonate is abundant. 
7. There develop in the secondary stem supernumerary leptome- 
