LYCOPOD1NEAE.LIGULATAE. 



295 



StrasburgerV account of the division in /. Ditriaci is, that the protoplasm of the 

 spore-mother-cell first divides into two and then into four parts, and this is followed 

 by the division of the nucleus of the mother-cell into four daughter-nuclei, one 

 of which goes to each macrospore. 



FIG. 240. Isoetes lactistris, macrosporangia in different states of development in 

 transverse section. A young state. B older state, the macrospore-mother-cell having 

 rounded itself off; 1 1 tapetal cells, TV trabeculae. C complete transverse section of a 

 macrosporangium in the same stage of development as S. Ma the individual macrospore- 

 inother-cells lying in the tissue (separated by the trabeculae). In Fig. A the sterile cells 

 (primary tapetal cells) cut off from the archesporium above the spore-mother-cells are 

 also indicated by tt. 



FIG. 241. Longitudinal section 

 through the lower sporangiferous 

 portion of a leaf of Isoetes lacustris. 

 the ligule, .7" the indusium (velum), 

 sp the sporangium (microsporan- 

 gium), TV the trabeculae, Gf vas- 

 cular bundles of the sporophyll. 



The development of the macrospores of Isoetes exhibits most significant 

 homologies with the macrospores (embryo-sacs) of the Gymnosperms and Angio- 

 sperms, as will be shown more fully further on. 



In many specimens of Isoetes from one locality, Lake Longemer in the Vosges, 

 a formation of vegetative shoots takes the place of the formation of sporangia 2 . A 

 shoot is formed at the position on the leaf where a sporangium is usually found, and 

 this shoot separates from the mother-plant and developes into a new plant. In 

 these plants the sexual generation is lost, and we have a case of apogamy as in the 

 prothallia of Ferns described above. The apogamy moreover presents itself in Isoetes 

 in various gradations ; it is sometimes complete and hereditary, in other cases some 

 leaves bear sporangia, others shoots. These apogamous plants appear to grow in 

 deep water. 



Histology. In the Selaginelleae, to which the following remarks chiefly refer, the 

 epidermis of the stem consists of long prosenchymatous cells without stomata ; the 

 lateral walls of the epidermal cells of the leaves are often delicately sinuous or they 

 have a variety of other forms ; like the same cells in the Ferns, they contain 

 chlorophyll, which appears in them and in the fundamental tissue of the leaves in a few 



1 Zellbildung u. Zelltheihmg, III Ed. p. 167. 



- Goebel. Ueber Sprossbildung auf Isoetesblattern (Bot. Ztg. 1879, No. i). 



