GROUP III. PTERIDOPHYTA : FILICIX.E. 377 



between these masses, the perinium usually terminates in a tuft of 

 delicate filaments. 



In the Marsileaceae the spores become invested by a perinium, 

 secreted by the epiplasm, consisting of an inner layer made up of 

 prisms placed with their long axes perpendicular to the surface of 

 the spore, and of an outer layer which is homogeneous in the case 

 of the microspore ; but in the case of the macrospore it is stratified, 

 swells up enormously on being wetted, and gives the cellulose- 

 reaction. 



In all cases the spore has its own proper coats, the exospore and 

 the endospore, of the usual constitution. It contains a mass of 

 granular protoplasm, with a nucleus, and encloses numerous starch- 

 grains, oil-drops, and proteid granules. 



The root is altogether absent in Salvinia ; in the other genera 

 the primary root is of but short duration, and the root-system con- 

 sists of numerous adventitious fibrous rootlets which have an apical 

 growing-point with a tetrahedral apical cell. In Azolla the root- 

 cap is but imperfectly developed, and in A. caroliniana it is 

 completely thrown off after a time. 



General Histology. In the Salviniacese the stem is monostelic ; 

 in the Marsileacese it is polystelic. 



On the whole the histology of these plants generally resembles 

 that of the allied homosporous Ferns, though in consequence of 

 their more or less aquatic habit the intercellular spaces of these 

 plants are more conspicuous, especially in Salvinia and in the root 

 of Pilularia where they form large air-chambers. 



The Einbryogeny of the Sporophyte. The early divisions of the 

 oospore are essentially the same as in the allied homosporous Ferns. 

 The individual peculiarities of subsequent development are briefly 

 as follows. In Salvinia the whole of the hypobasal half goes to 

 form the foot, no primary root being developed ; of the four 

 epibasal octants, one gives rise to the growing-point of the stem, 

 and two to that of the first leaf or cotyledon, and from the basal 

 region of these octants a transverse layer of cells is cut off which 

 eventually elongates forming a cylindrical hypocotyl, sometimes 

 termed the caulicle ; the cotyledon is termed the scutiform leaf on 

 account of its form and mode of attachment ; the young stem first 

 produces one or two isolated foliage- leaves, and then the regular 

 development of the whorls of two foliage-leaves and one water-leaf 

 (see p. 373) begins. In the other three genera, two of the epibasal 

 octants give rise to the first cotyledon, one to the growing-point of 



