248 
METHODS IN PLANT HISTOLOGY 
situations on the borders of ponds, along ditches, or on moist meadows. 
While the plant is very small, it has large spores. Several of the 
tropical species are common in greenhouses, and they fruit abundantly. 
Vegetative structure. —Growing points and root-tips are easily 
cut in paraffin. In most species, the older parts of the stem are too 
hard and brittle to cut in paraffin and are too small to cut well free¬ 
hand. It might be worth while to try hydrofluoric acid or the cellulose 
acetate method when that method becomes developed. At present, 
patience and a sharp knife seem to be the only reliance. Some of 
the tropical species, like Selaginella Wildenovii, have stems nearly as 
large as a lead pencil, with polystelic structure, and are not hard to 
cut. The vascular cylinder is an exarch protostele or, when poly¬ 
stelic, each bundle is an exarch protostele. It is exceptionally easy 
to get a brilliant, differentiated stain when once the sections are cut. 
The strohilus. —Very young strobili cut easily in paraffin, but 
after the megaspore coats begin to harden, there are few objects 
which make more trouble than the strobili of Selaginella. For stages 
up to the young megaspores, fix in chromo-acetic acid, with or without 
the addition of a little osmic acid; but for later stages use hot corrosive 
sublimate acetic acid in 50 per cent alcohol. If this is not available, 
use formalin, acetic acohol. Sometimes sections of the difficult 
later stages stay on the slide with Mayer’s albumen fixative. Even 
stages like that shown in Figure 79 should always stay on with Land’s 
fixative. 
The strobili of most species are square in transverse section. 
To get longitudinal sections showing the relations of sporangia, sporo- 
phylls and axis, cut diagonally, from corner to corner, never parallel 
to the flat side. For archesporial cells, use iron-haematoxylin; for 
young megaspores and the development of spore coats, use safranin, 
gentian-violet, orange; for later stages, use safranin and light green. 
The gametophytes. —In most cases the spores germinate while 
still within the sporangium and, in some cases, like Selaginella apus , 
the female gametophyte develops up to the archegonium initial stage 
before shedding. If strobili of this species at the stage shown in 
Figure 79 be broken off and laid down on moist ground so as to keep 
the sporangium moist, dehiscence may not be vigorous enough 
to discharge the megaspores; but development of archegonia and 
antheridia will continue, fertilization will take place, and an embryo 
will be developed while the megaspore is still inclosed within the 
