ISOETES. 



[ 443 ] 



ISOETES. 



discoverable in the interior of the bud 

 twelve months before they became fully 

 developed ; the sterile originate in spring 

 and the earlier part of the summer, the 

 fertile in the autumn, wliile stunted fertile 

 leaves appear even in the winter. If a 

 vigorous leafy plant be examined, it will be 



Fig. 376. 



Isoetes setacea. 

 Natural size. 



found to have a few sterile leaves outside, 

 then a circle of leaves with oosporanges, 

 next a circle of anthero-sporanges, and in 

 the centre of the bud sterile leaves closing 

 the annual cycle. The sporanges are some- 

 what plano-convex longish-oval cases, with 

 transverse processes forming imperfect septa, 

 dividing them into several chambers (hg. 

 378). The cases are sheathed by a mem- 

 branous expansion of the base of the leaf 

 (fig. 377), to which they are adherent by 

 the back (fig. 378) ; the septa arise 

 opposite the point of attachment at the 

 back, and, spreading out, join the front wall. 

 The ditiereut contents of the sporanges are 

 evident before they open, those with the 

 small spores (antJierosporanges) having a 

 smooth face, those with large spores {oospo- 



ranges) heing rendered tubercular from the 

 protrusion of the wall by the underlying 

 bodies. The wall of tlie capsule is mem- 

 branous and has no regular dehiscence, the 



Fig. 377. 



Fig. 378. 



Fig. 379. 



Isoetes setacea. 



Fig. 377. Base of a detached fertile leaf, seen in face. 

 Magn. .5 diams. 



Fig. 378. Vertical section, from back to front of ditto. 

 Magn. 10 diams. 



Fig. 379. Horizontal section, oosporange with macro- 

 spores. Magn. 10 diams. 



spores escaping by decay of the membrane 

 in front. 



The sinalle?- s/jo?ts resemble pollen-grains; 

 they are usually of the shape of quarters of 

 a globe, more rarely tetrahedral, with an 

 outer coat presenting ridges at the angles, 

 and an inner which is a rounded sac. The 

 outer coat is finely dotted in /. laeustris. 

 The large spores are at first of a tetrahedral 

 form with rounded angles, but when ripe 

 they become globular. The delicate inmost 

 layer is enclosed in a thick exospore com- 

 posed of three layers: — the innermost of mo- 

 derate thickness, brown colour, and glassy 

 consistence, exhibiting strife and three 

 strong ridges converging to a point at the 

 angle where the spore meets its three sister 

 spores ; the next coat is thinnish, and of 

 granular character and yellow colour ; the 

 outermost is a clear and gelatinous layer : 

 the outer two follow all the markings of 

 the glassy coat, and are especially thick 

 over the three ridpres. 



