Osborn . — Some Observations on Isoetes Drummondii , A.Br. 47 
specimen by two triangular imbricating scales, only the outer of which was 
sclerized when the specimen was collected. For sectioning it was necessary 
to remove the tough sporophyll bases and sporangia from the upper surface. 
Their attachment is shown by the mucilage tissue produced at their bases. 
The whole of the living parenchyma is packed with starch, and the stock 
at this stage is merely a perennating organ, its vegetative activity being 
suspended. 
Fig. 6, c, shows the condition of such a stock about six months later, 
when the leaves and roots of a new vegetative season have appeared, and 
the sporangia of a previous one are being shed. The development of new 
leaf- and root-bearing surfaces has caused a rupture of the continuous 
sclerenchyma layer. This new growth has taken place at the expense 
of the starch, &c., stored in the outer region of the stock. This is now 
almost depleted of plastic substances at its periphery. The development 
of the cortex in the ‘new leaf- and root-bearing regions forces the older 
parenchyma outwards, since the amount of elongation of the axis between 
leaf- and root-forming meristems is negligible. But, since the whole 
structure is subterranean and subjected to pressure by the soil on all sides, 
the lateral expansion at the centre causes distortion and crushing of the 
moribund distal portion, which, because the cells have ceased to grow and 
keep pace with the increasing circumference, becomes torn asunder and 
forms projecting lobes. These distal portions soon become cut off by 
sclerenchymatous tissue (cf. Fig. 6, a ) and yet another cap is added to 
the lobe. 
Thus it is seen that each annual set of caps represents the whole of the 
leaf- and root-bearing cortex of one growing season. 1 The annual desqua- 
mation of these caps follows because of the sharp alternation of vegetative 
and resting seasons imposed upon a plant showing the peculiar growth 
mechanism of an Isoetes stock. The development each season of a starch- 
packed resting structure, upon which the vegetative apex is born, is 
analogous to the seasonal production of a corm by such a plant as Hypoxis 
(Iridaceae), with which Isoetes Drummondii is found associated in the field. 
But the analogue must not be pressed too far, for Hypoxis has the usual 
conical apical growing region of most plants and not an invaginated one as 
in the Isoetes . Consequently its stem elongates appreciably in the course 
of each growing season, while new roots are annually formed adventitiously 
around the base of the stem. Hence the old corm, composed of exhausted 
storage parenchyma, becomes crushed below the growing plant each year. 
But in Isoetes Drummondii and other species of the genus root-production 
is limited to certain lines on the lower surface of the stock, which enlarges 
each growing season to allow of their expansion. This, coupled with an 
1 Lang, W. II. 1 loc. cit. Von Mohl, H. : Vermischte Schriften botanischen Inhalts, 1845, 
pp. 1 2 2-8. 
