364] TRANS. ST. LOUIS ACAD. SCIENCE. 7 
the transverse section is rather circular, but in the 2-lobed ones 
it is usually oblong or often somewhat rectangular, narrower in 
the direction of the grooves and wider in the opposite direction ; 
the vertical section of the trunk shows a thickness of from i to 6 
lines, and the transverse diameter a width of a few lines to more 
than one inch. In the centre of the trunk we find a small ligne¬ 
ous body, fibres from which enter the leaves and the roots. The 
mass of the trunk is a white parenchymatous or rather cortical 
tissue, the cells of which are filled with starch. The growth pro¬ 
ceeds from the central ligneous body outwardly in two or three 
directions, corresponding to the two or three lobes, so that these 
lobes would spread laterally if their enlargement were not limited 
by the decay of the older (the preceding year’s?) parts. We thus 
find that at the period of the most vigorous growth, about the 
beginning of fructification, the extreme lateral portion of the lobes 
becomes discolored, brownish, atrophied, and at last black, and 
is separated from the living tissues by a distinct line of demarca¬ 
tion, and at last generally falls off at the end of this or the be¬ 
ginning of the next season as a black mould-like mass. In some 
species, e g. /. lacustris , and in colder climates the atrophied 
cortical parts continue to cohere for several seasons, and in the 
Mediterranean 1 \ Hystrix they do not seem ever to be detached, 
so that the trunk of this species reaches a larger size than any 
other. 
The decaying portions are pushed obliquely upwards when 
the base of the trunk grows faster than the upper part (often in 
I. Engelmanni , and much more so in the Australian /. tripus), 
or horizontally outward (the ordinary case), or downward when 
the upper or leaf-bearing part expands more than .the lower, root¬ 
bearing part. This last is the case in I. Hystrix , where the 
dead parts are turned downwards. 
As the growth of the trunk takes place from the centre out¬ 
ward, the roots, originating from the youngest parts, start from 
the groove itself; and fresh and living, whitish, ones are only 
found in or near this groove : as they get older they are pushed 
to the sides, and finally die, becoming brown and black. The 
mass of roots found on Isoetes specimens are mostly the entan¬ 
gled dead fibres, which, by the way, often conceal spores of the 
previous year, and therefore must be carefully examined when 
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