224 Methods in Plant Histology 



three of which are common in the range of Gray's Manual. Of 

 these three, Selaginella apus is best for sections. Several of 

 tropical species are common in greenhouses and they fruit abun- 

 dantly. 



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. Patience and a sharp knife seem to be the only reliance. 

 Some of the tropical species, which have stems as large as a lead 

 pencil and not very hard, are best for sections. The vascular cylin- 

 der is an exarch protostele and it is exceptionally easy to get a sharp 

 differential stain when once the sections are cut. 



The strobilus. 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 formalin 

 alcohol, and fasten the sections to the slide with Land's fixative. 

 Even the oldest stages can be cut in paraffin (Fig. 74). 



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 paral- 

 lel 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 some species, the megaspores and micro- 

 spores germinate and even develop up to the egg and sperm stage 

 while still retained within the sporangia (Fig. 74). For such stages, 

 if the strobilus is fixed entire, use formalin alcohol; if the megaspores 

 are removed, use chromo-acetic acid. 



To make cultures, shake the spores out as in case of ordinary 

 fern prothallia, or scatter the whole strobili over the soil. The 

 female gametophytes within the old spore coats generally orient 

 themselves in the paraffin, the base of the spore being clown and the 

 archegonium end of the gametophyte being up. 



