MICROSCOPIC OBJECTS. 38 



Fig. 75. Transverse Section of the Stem of a Lycopodium 

 (Lycopodium iiisequalifolium) , x 20. Acrogen. 



A section of this club-moss has been figured to illustrate 

 the structure of an acrogenous stem, as differing from the 

 exogenous and endogenous stems shown in figs. 59, 71, 72, 

 73, &c. 



On the outer part of the stem a thin layer of cells, which 

 have been thickened by secondary deposit and dotted, will 

 be seen and inside this formation a loose layer of cellular 

 tissue, surrounding bundles of scalariform tissue. For another 

 example of an acrogenous stem see fig. 11. Lycopodiums 

 appear to be closely allied to some Ferns as regards the 

 position &c. of their seed-vessels. The creeping Selayinella 

 of our Wardian cases belongs to this Natural Order. 



The transverse and other sections of this plant may be 

 mounted in fluid or glycerine jelly. 



Fig. 76. Fructification of a Fern (Polypodium vulgare) , 



x40. 



From the various species of British and foreign Ferns and 

 Mosses some of the most beautiful and instructive objects 

 may be taken by the microscopist and botanist for their 

 cabinets. The present drawing shows the naked sori for 

 which this genus of Ferris is remarkable, as the sori, or groups 

 of seed-vessels as they may be called, of Ferns are generally 

 covered with a cellular membrane or skin of different shapes. 

 Most species have a reniform or kidney-shaped covering, or 

 indusium as it is called (see fig. 77). The masses of sori 

 generally occur on the back of the fertile fronds, although in 

 some cases they are found on the margins, as in the Maiden- 

 hair Fern (Adiantum); and sometimes they are placed alone 

 upon stalks, as in the Osmund a Fern (fig. 79), in which case 

 they form capsules. 



This object is best mounted in a dry opaque cell. 



