Knox: Stem oF: IBERVILLEA SONORAE 33a 
endodermis. Outside of the endodermis there are two layers of 
chlorenchyma with autochthonous starch (FIGURE 2, ch), and then 
two layers of angle-collenchyma (co/), chlorophyl-free, bordered 
by a large-celled epidermis. There are occasionally stomata, but 
they show no special adaptations to desert conditions, nor is the 
cutin-layer unusually thick. 
There remains to be mentioned a system which has been ad- 
mirably treated by Fischer (17), namely the sieve-tubes which are 
found scattered singly or in groups throughout the pericycle and 
the cortex. The discovery of supernumerary sieve-tubes just in- 
side the stereome-ring was made by Sanio (4) in 1864 in Cucumis 
sativus. DeBary (6) found them in the same position in a number 
of other species, but Fischer added two new categories to those of 
Sanio andde Bary. To the study of their development and phys- 
iology he devotes 109 pages. He finds sieve-tubes with com- 
panion-cells and ‘nebencellen” without as well as within the 
stereome-ring. There are also horizontal series which he desig- 
nates commissural sieve-tubes and sieve-bundles which connect with 
the leptome of the bundles and with each other. These sieve-tubes 
are to be seen only in the very young tips, for they function dur- 
ing the period of elongation, and owing to the pressure of the 
growing tissues about them soon lose their typical structure. 
Fischer describes their progressive phases of development accord- 
ing to Sachs’ phases of growth, and treats especially of the meri- 
stematic period of elongation. The first sieve-tubes to appear in 
the stem, which function before any of the others, lie on the edge 
of the outer procambial strands between the more typical phloem 
region and.the mother-tissue of the stereome-ring. At this stage 
the procambial strand borders closely on this mother-tissue, but 
later by the development of parenchyma they become more iso- 
lated from the bundle and stand out in the pericycle. In /éervillea 
they are very prominent, with large conspicuous sieve-plates. 
Following these there appear first the sieve-tubes of the outer 
leptome, and with them the subepidermal ectocyclic sieve-cells and 
the ectocyclic cells which are found among the collenchyma. 
The inner leptome comes next in order, and just before the end of 
the meristematic elongation the endocyclic sieve-tubes mature. 
The commissural strands may originate with, but never before, the 
