ISOETES. 



[ 369 ] 



ISOETES. 



membranous and has no regular dehiscence, 

 the spores escaping by decay of the mem- 

 brane in front. 



Fig. 380. 



Fig. 381. 



Fig. 382. 



Isoetes setacea. 

 Fig. 380. Natural size. 

 Fig. 381. Base of a detached fertile leaf, seen in face. 



Magn. 5 diams. 

 Fig. 382. Vertical section, from back to front of ditto. 



Magn. 10 diams. 



Fig. 383. 



Isoetes setacea. 



Fig. 383. Horizontal section, oosporange with macro- 

 spores. Magn. 10 diams. 



The smaller spores 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 I. lacustris. 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 composed of 

 three layers; the innermost of moderate 

 thickness, brown colour, and glassy consist- 

 ence, exhibiting striae 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 ridges. 



The contents of the microspores are at 

 first merely granular protoplasm. About a 

 month after they are scattered from the 

 sporange, the primordial utricle becomes 

 divided into two or four portions, which 

 form cells, in each of which again are deve- 

 loped two vesicles, each producing a filament 

 coiled up spirally. The spores swell, the 

 daughter-cells burst,and the lenticular vesicles 

 escape ; the latter then open and emit the 

 spiral filaments, which are found to be co- 

 vered with cilia on the anterior turns of the 

 spiral, by means of which they move actively 

 through the water. They are the spermato- 

 zoids. 



The macrospores, when they escape from 

 the sporange, contain only protoplasm with 

 oil-globules. In the course of a few weeks, 

 the internal cavity of the spore begins to 

 exhibit a development of cellular tissue, by 

 which it is subsequently filled up. This is 

 the prothallium. At the same time, the in- 

 ternal coat increases in thickness, and exhi- 

 bits several layers. The increase of size of 

 the prothallium causes the spore-coat to 

 burst at the apex where the three ridges 

 meet, so that three triangular valves turn 

 back, exposing the prothallium. On this 

 are developed the archegonia, the first on 

 the apex in the central point where the three 

 points of the spore-coat meet. If this is not 

 fertilized, others are produced around it. 



2B 



