78 SUMMARY OF CURRENT RESEARCHES RELATING TO 



upwards, and finally forced against the embryo. The hood-like 

 fibrous layer which is closely applied to the floating apparatus, is 

 turned over, and surrounds the foot of the embryo like a collar. 

 Shortly afterwards the embryo detaches itself from the macrospore ; 

 the margins of the scutellum become broader, and then lie on the 

 surface of the water in the form of cups or scales. 



The strongly refractive bodies previously observed by others 

 between the indusium and epispore, are, according to the author, 

 Nostoc-eells, which find their way into the crevices between the scu- 

 tellum and the young leaves when the apex of the embryo appears 

 outside the epispore. 



Development of the Sporangia and Spores of Isoetes.* — On the 

 disputed point whether the sporangia of Isoetes spring from super- 

 ficial or from deeper lying cells, E. Mer considers that he has 

 demonstrated the latter from the case of sterile leaves which are the 

 result of the abortion of the sporangia at various stages. 



In the earliest stage of development of the sporangium, while the 

 leaves are still in vernation, it is not connected with the leaf by a 

 pedicel ; the tissue is, on the contrary, homogeneous, composed of 

 young, very delicate, polyhedral cells, with no trace of trabeculae or 

 envelope. The pedicel is afterwards formed by expansion of the 

 lateral parts. The cells of which it is composed difier from those of 

 the rest of the organ ; they are elongated horizontally, are polyhedral, 

 with very acute angles, and enclose starch. The macrosporangia and 

 microsporangia can be distinguished even at this period. Among the 

 cells of the macrosporangium appear radiating rows of cells, similar 

 to those of the pedicel, which are the young trabecule ; the external 

 envelope becoming at the same time differentiated. 



In the second stage the mother-cells of the macrospores increase 

 in size, and contain vacuoles, growing at the expense of other cells 

 which decrease in size and at length entirely disappear. The 

 nutritive tissue is finally confined to one or two rows of cells situated 

 at each side of the trabeculae, which no longer contain starch. 



In the third and final stage the mother-cells of the macrospores 

 divide into tetrahedra ; the macrospores become isolated, and float in 

 the empty space between the trabeculae. The mother-cells of the 

 microspores cannot be made out till a later period than is the case 

 with the macrospores. 



In the primitive meristem, from which are developed the macro- 

 and microsporangia, three tissues are speedily difierentiated : viz. a 

 formative tissue destined to produce the mother-cells ; a nutritive 

 nitrogenous tissue, which is absorbed at the expense of the mother- 

 cells ; and an amylaceous nutritive tissue intended to supply the 

 mother-cells with nutriment. 



M. Mer found that the supply of food-material caused a re- 

 markable diff'erence in the development of Isoetes lacustris, of which 

 he accordingly distinguishes four forms. An abundant supply of 

 food is necessary for the formation of the macrosporangia, an 



* Bull, Soc. Bot. France, xxxviii. (1881) pp. 72-6, 109-13. 



