412 PART III. THE CLASSIFICATION OF PLANTS. 



rounding the sori soon becomes disorganised, as also tne walls of 

 the sporangia. 



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

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

 consists 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 Salviniacea3 the stem is monostelic ; 

 there is an axial, closed, concentric, vascular cylinder, without any 

 pericycle but with a two-layered endodermis of which the outer 

 layer has the characteristic markings in the radial cell-walls. 



In the Marsileaceae the stem is polystelic and gamostelic ; there 

 are five steles which are distinct at the growing-point but fuse 

 further back forming a hollow cylinder enclosing some ground- 

 tissue which looks like, but is not, a pith. 



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. 



Azolla presents a case of symbiosis which resembles that of 

 Anthoceros (p. 353). On the upper surface of the dorsal lobe of each 

 leaf, near the base, a pit is formed which has a narrow aperture 

 and is occupied by Nostoc-filameiits (Anabaena) amongst which 

 branch hairs which are developed from the epidermal cells lining 

 the pit : these filaments also occur in the macrosporangiate 

 indusium, having entered before the indusium becomes closed. 



The Embrijogeny of the SporopJtyte. 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 



