PTERIDOPHYTA 



813 



increases in quantity, and when the spore is about half grown, a section through 

 it shows a thin layer of cytoplasm in contact with the wall, and containing 

 numerous free nuclei (Fig. 277, A, B). This closely resembles the early stages 

 in the development of the gametophyte among the lower Seed-plants. The cell- 

 formation begins at the apex of the spore. Between the free nuclei there are 

 developed connecting fibrils, forming more or less evident bundles of fibres, and 

 between each pair of nuclei a cell-wall is developed, so that the protoplasmic layer 

 is divided up into (usually) hexagonal cells. The subsequent nuclear divisions 

 are followed by the formation of cell-walls', and there is then formed in the apex 

 of the spore a flat mass of tissue, upon which the archegonia are developed. 

 About this time the spores are ripe, and when discharged from the sporan- 

 gium, continue to develop the gametophyte, which finally breaks open the spore- 

 membrane, and exposes the tapper part of the gametophyte with the archegonia. 

 The latter are small, and the neck canal-cell is undivided. 



The Embryo 



A- 



The embryo (Fig. 277, H, I) is much like that of Lycopodium, but the first 

 division occurs before the egg has increased much in size, and the suspensor is 

 much longer. A definite api- 

 cal cell is present in the stem- 

 apex, and the two cotyledons 

 arise on either side of it. A 

 foot is developed, and the root 

 arises between it and one of 

 the cotyledons. The first divi- 

 sion of the primary shoot 

 seems to be a true dichotomy. 

 The elongation of the suspen- 

 sor forces the young embryo 

 into the lower mass of tissue 

 of the gametophyte, and this 

 is destroyed by the embryo in 

 its growth. When it emerges 

 from the spore (Fig. 278, B), 

 it very much resembles a typi- 

 cal dicotyledonous seedling. 



THE MATURE SPORO- 

 PHYTE 



In the creeping stems, 

 the structure is mono- 

 stelic, but this may be 

 replaced in the upright 

 shoots by a bistelic or 

 polystelic structure. The 

 individual bundles are 

 concentric in structure, 

 with usually two proto- FIG. 280. Selaginella Kraussiana. A, section of 

 xvlein grouns (Tiff 280 8tem ' showin S th e two vascular bundles sus- 

 , g ' f * pended in the central lacuna (X 75). B, a single 

 B). The apical growth of bundle (X 450) ; x, trache* ; s, sieve-tubes. 



