Scott and Hill.—Structure of Isoetes Hystrix. 429 
started, each lying nearer the median line of the furrow than 
its predecessor. Hence we may at any time find new roots 
arising near the centre of the basal surface ; these belong to 
new rows which have only just started. With reference to 
the whole stem, the succession of the roots in each series is 
thus acropetal, while the succession of the series themselves 
is centripetal, with reference to the centre-line of the furrow 
to which they belong. In Figs. 11 and 12 the succession 
Fig. 12. Diagram of the root-succession, as shown in a transverse 
section from the base of the stem. For description see text. 
within each series is represented by numbers, No. 1 being the 
youngest root shown in its series ; the succession of the series 
themselves is represented by letters, A being the youngest 
series of its set. There will usually be two, about equally 
young series, one on each side of the median line of the 
furrow. It would rarely happen that the same series could be 
traced through its whole course. The older series would 
usually be obliterated near the base of the stem, while the 
younger series would be incomplete towards the apex. It 
