Scott and Hitt.—Structure of Isoetes Hystrix . 439 
with the endodermis, and at the opposite side from the proto- 
xylem (Fig. 32). There is thus no distinction between the 
monarch strand of the Isoetes root, and an ordinary collateral 
bundle of the endarch type. 
Where dichotomy of the root takes place, the plane of 
division passes through the protoxylem and protophloem. 
A stele at the base of a dichotomy is shown, in transverse 
section, in Fig. 34. As the two strands separate from each 
other they turn through an angle of 90°, so as to direct their 
protoxylem-groups towards each other. The process is pre¬ 
cisely the same as in the roots of a Selaginella (Van Tieghem, 
1871, p. 99, PI. 5, Figs. 25 and 26), or in the rootlets of a Stig- 
maria. We have not examined the minute structure of the 
phloem of the root, which appears to resemble that of the leaf. 
There are certain statements in the literature as to the roots 
of Isoetes which are not confirmed by our observations. Prof. 
Van Tieghem (1891, p. 1429) states that ‘while the root is 
descending in the cortex, the structure of its central cylinder 
is binary, and at first normal ; the diametral band of xylem is 
parallel to the central cylinder of the stem. But gradually 
one sees the phloem-bundle situated on the side towards the 
axis of the stem, become more and more reduced, and finally 
disappear, while the diametral band becomes applied to, and 
concentrated towards the pericycle V We can find nothing 
answering to this in Isoetes Hystrix. 
However near the stele of the stem the root-trace is ex¬ 
amined, its phloem is always limited to the side remote from 
the wood of the stem. Further, in this region, just as in the 
more distal parts of the root, the development of the xylem 
proves that its structure is monarch throughout. Fig. 33 
shows a young root-bundle cut close to its junction with the 
stelar wood of the stem. There is manifestly one protoxylem- 
group only, and the phloem, though as yet but little differen¬ 
tiated, is clearly limited to the side remote from the proto- 
1 This account differs from that on p. 690 of the same work, where the normal 
structure in Isoetes , like that in Selaginella , is explained by abortion of one xylem- 
bundle, and fusion of the two groups of phloem. 
