478 EXPLANATION OF THE FIGURES. 



PIG. 



25*. Front view. \ Tlie same i etters repr esent the same parts. 



25 c . View from above. 5 1 ' 



2G. A germinating spore, whose prothallium is broken through by the first leaf 

 (not by the elongating axis of the embryo), X 50. 



27. Spore with prothallium and embryo after the elongation of the axis of the 



latter, X SO. 

 27*. The terminal bud of the latter embryo, x 300. 



28. A more developed germ-plant, with spore attached, in longitudinal section, 



X 150. 



29. Terminal bud of a similarly developed germ-plant, seen from above. Near the 



third leaf, which is still closely folded (to the left in the figure) is seen the 

 end of the principal axis ; underneath it are three of its delicate forks, 

 which are developed iuto the so-called roots. 



PLATE XLVI. 



ISOETES LACTJSTEIS.* 



1. A large spore, a fortnight after sowing, and after soaking for several hours 



in glycerine, seen from above. The first-formed cells of the prothallium 

 appear spread over the inner wall. 



2. Longitudinal section of prothallium four weeks after sowing, x 40. 



3. Portion of the apex of a prothallium cut through longitudinally, with two 



archegonia still in process of development, x 300. 



4. Archegonia ready for the separation of the angles of contact of the upper 



three double pairs of cells, in longitudinal section, x 300. 



5. 6. Longitudinal sections of archegonia readv for impregnation, x 300. 



7. A ripe microspore seen from above (perpendicular to ^the transverse dia- 



meter) X 500. 



8. Lateral view of a microspore four weeks after sowing. The mother-cells 



of the spermatozoa are formed, x 500. 



9. Microspore three weeks after sowing. By rolling the spore under the cover- 



ing glass the exosporium has burst and is pushed on one side. The 

 mother-cells of the spermatozoa entirely fill the inner cavity of the spore. 

 X 500. 

 10. A small spore, burst, four weeks after sowing. It has already sent forth 

 several spermatozoa; one of them, still partly enclosed by the mother- 

 cell, is seen within the latter in active motion. 



* Plates XLVI to LIII relate to Isoetes lacustris, and the letters a, b, &c, 

 have the following significations : 



a Archegonium. ac Central cell, ao Mouth of the same, ax End of the primary 

 axis of the embryo, cb Camium. ct Back of the stem, e Embryo, fr Leaves ; 

 fr l the first,//- 2 the second, leaf of the germ-plaut, and so forth, fa Vascular 

 bundle. In some cases the vascular bundles passing to the leaves are merely 

 represented bjfr x &c, and those passing to the roots by r 1 , r 2 , &c. (so also in 

 fig. lb of PI. XLIX). g Terminal bud ; gc the apical cell of the latter. I Woody 

 mass; Isp the upper, and hi/the, lower, portion of the latter. Ig The supple- 

 mentary shoot of the fore-side of the leaf covering the base of the scale. rBoot. 

 re The cell of the first degree of the root, v Sheath of the base of the root. 



