386 PART III. THE CLASSIFICATION OF PLANTS 



ment of several distinct sporangia in the place of one, a tendency which is 

 more clearly marked in the Marat tiacese. 



With regard to the histology of Isoetes, the monostelic stem has a solid cen- 

 tral mass of vascular tissue formed by the collateral bundles coming from the 

 leaves. The wood consists of very short reticulated and spiral tracheids with 

 scattered parenchymatous cells, and is surrounded by a layer of transparent 

 tissue, consisting of shortly prismatic cells with broad and delicate pits, which 

 represents the bast. From the lower surface of the vascular mass, opposite the 

 furrows of the stem, are given off the bundles which go to the roots. 



The stem undergoes slow growth in thickness, effected by a merismatic layer 

 situated externally to the layer of prismatic cells, and only interrupted by the 

 passage of bundles from the axial vascular cylinder of the stem to the leaves 

 and roots. The merismatic layer gives rise to tissue both internally and exter- 

 nally. The internal tissue consists of vascular tissue, and is formed in relatively 

 small quantity ; the external tissue is bulky, and consists of pareuchymatous 

 cortical cells. This cortical tissue is not produced uniformly all round, but on 

 two or three sides, according to the species. It is to this that the lobed and 

 furrowed appearance of the stem is due. 



The leaves are characterised by the presence of four longitudinal rows of large 

 intercellular air-spaces, extending from one end to the other in the mesophyll, and 

 by the presence of a single median vascular bundle. In the aquatic species there 

 are no stomata in the epidermis ; the intercellular spaces are situated deeply 

 within the tissue, and there are no hypodermal strands of sclerenchymatous 

 tissue. In the amphibious and terrestrial species there are stomata, the inter- 

 cellular spaces are superficial (immediately beneath the epidermis in terrestrial 

 species), and there are longitudinal hypodermal strands (4-6) of sclerenchyma- 

 tous tissue which give rigidity to the leaf. 



The root has an essentially diarch stele, which becomes, however, monarch 

 as in Oplnoglossum vulgatum. It is surrounded by a well-marked bundle- 

 sheath. The cortical tissue is clearly marked, by intercellular spaces, into an 

 inner and an outer region. 



The development of the embryo-sporophyte begins with the formation of the 

 basal wall, which is obliquely transverse to the long axis of the archegonium, 

 and divides the oospore into an epibasal and a hypobasal half. Both these 

 cells undergo division into two, by the formation of a wall, the transverse 

 wall, at right angles to the basal wall, so that the embryo now consists of four 

 quadrant-cells ; and this appears to be generally followed by the formation of 

 a vertical wall, the median wal 1 , at right angles to the two preceding, so that 

 the embryo comes to consist of eight octant-cells. Owing to the difficulty at 

 distinguishing the growing-point of the young stem, there is still some uncer- 

 tainty as to the exact relation of the members of the embryo to these octants, 

 but it appears to be probably somewhat as follows : The first leaf (cotyledon) 

 arises from the two upper epibasal octants ; the growing-point of the stem, to- 

 gether with the first root, arises from the two lower epibasal octants ; the four 

 hypobasal octants give rise to the large foot. If this be so, then the first root, 

 springing as it does from the epibasal half of the embryo, must be regarded, not 

 as a true primary root, but as an adventitious root, a view which is supported 

 by the fact that the origin of the growing-point of the root, though not abso- 



