496 MOSSES AND FERNS chap. 



arranged in four rows, two lateral ones, composed of large 

 leaves, and two dorsal rows of smaller ones (Fig. 260). In 

 the homophyllous forms the sporophylls differ but little in 

 appearance from the ordinary leaves, but in the heterophyllous 

 ones they are smaller than the other leaves, and always 

 arranged spirally about the axis, forming a strobilus much like 

 that of Lycopodiuni, but usually less conspicuous. Commonly 

 the lowest or oldest sporangium is the macrosporangium, and 

 contains four spores ; the younger ones, which may continue to 

 form for a long time, are always microsporangia, and are very 

 similar in appearance to those of Lycopodiuni. 



The Stem 



The apex of the stem has the form of a cone, whose 

 summit, in most species examined, is occupied by a single 

 apical cell. This, in 5. Kraussiana (Fig. 261), is of the "two- 

 sided " type, and segments are regularly cut off only from the 

 lateral faces. From inner cells of the segments are derived 

 the two vascular bundles (steles), which are found in the fully- 

 developed stem, but their limits are difficult to trace in the 

 small-celled meristem at the apex. In other species there is 

 great variation in the character of the apical meristem. Thus 

 in 5. Martensii, according to Treub,^ the apical cell of the 

 older shoots may be either a two-sided one, like that of vS. 

 Kraussiana, or it may be tetrahedral, like that of Equisetum 

 and most Ferns. In the younger branches, however, a four- 

 sided cell, like that Pfeffer describes for the embryo previous to 

 the first forking of the stem, is always present, but is later 

 replaced by the two-sided or tetrahedral form. Strasburger ^ 

 found in .S. Wallichii regularly two apical cells, and several 

 species, e.g. S. arborescens, S. spinosa, show the same type of 

 apical growth as Lycopodiuni. 



Sections of the stem apex, parallel to the plane of the 

 leaves, frequently show the formation of the branches (Fig. 261, 

 B). It is quite evident that the branch arises as a lateral out- 

 growth of the stem apex, which retains its original central 

 position for some time. The apical cell of the branch is not 

 established until the latter is very evident. By the rapid 

 growth of the branch it may very early force the main axis 

 1 Treub (i). ^ Strasburger (7). 



