436 Scott and HilL—Struchire of Isoetes Hystrix. 
of A. Braun (1863, p. 569), lying between the young sporangium 
and the ligule. It is certainly not the case in I. Hystrix 
that it arises as ‘ a sterilized portion of the sporangium/ 
as Wilson Smith maintains (1900, p. 242). The first growth 
from the saddle is in the upward direction ; to form the 
labium of A. Braun (Fig. 26 ). The upward growth soon ceases, 
and the velum develops in the downward direction, keeping 
pace with the growth of the sporangium, around which it 
forms a complete pocket, open only by a narrow crevice 
at the lower end (Fig. 26). This opening was called the 
micropyle by Braun. The complete velum is characteristic 
of the terrestrial species of Isoetes . As the velum is perfectly 
continuous with the leaf at the sides, as well as at the 
upper end, its development must take place by intercalary 
growth, proceeding pari passu with that of the leaf-base and 
sporangium. 
Practically the whole growth of the leaf is intercalary, the 
growing region being at first localized at the level of the 
sporangium, and subsequently extending higher up, above 
the insertion of the ligule. It is only at the first origin of the 
leaf that there is any apical growth. 
The development of the persistent, spinose leaf-bases of Isoetes 
Hystrix was fully worked out by Alex. Braun (1863, p. 578), 
whose account we have simply to confirm. The facts are briefly 
these. A transverse ring of tissue at the base of the leaf, extend¬ 
ing up to the lower edge of the sporangium, greatly thickens 
the walls of its cells, so as to form a continuous armoured belt. 
The sclerotic belt reaches the dorsal and ventral surfaces, and 
may include the lower part of the velum, but it does not 
extend out into the wings, the thin part of which withers 
away. At certain points the sclerosis extends further upwards, 
forming the spines. These may be four in number: there 
are two lateral horns, formed from tissue at the insertion 
of the wings, a ventral tooth formed partly from the velum 
and partly from tissue on the opposite side of the sporangium, 
the remains of which are thus partially enclosed by the 
sclerotic region, and lastly there may be a dorsal tooth, which 
