BRYOPHYTES— HEPATICAE 279 



52° C. parafRn, Watts safety razor blade, and a room temperature of 

 — 2° C, this microtome cuts translucent ribbons of root-tips at 0.5 fx. 

 The ribbons should not stretch more than 20 per cent when warmed 

 on the slide. It is a good thing to practice cutting ribbons at 1 and 

 0.5 n, since the extreme care, so absolutely necessary for these very 

 thin sections, will develop a technique which will make 2 and 3 /n sec- 

 tions seem easy. 



If sperms are found escaping, transfer them to a small drop of water 

 on a clean slide, invert the drop over a 1 per cent solution of osmic acid 

 for ten seconds, allow the drop to dry up, pass the slide through the 

 flame 2 or 3 times, as in mounting bacteria, and then stain sharply in 

 acid fuchsin. This should show the general form of the antherozoid, 

 and will usually bring out the cilia. 



The archegonia. — The methods for archegonia are practically the 

 same as for antheridia. Too much stress cannot be laid upon the im- 

 portance of carefully selecting the material. Use very small pieces, 

 and, before placing them in the fixing agent, trim them to such a shape 

 that the position of the archegonia will be known accurately even after 

 the pieces are imbedded in parafRn. Since air is likely to be caught be- 

 tween the perigynium and the archegonium, it is worth while to use 

 an air pump or an aspirator as soon as the material is put into a 

 chromic fixing agent. With the formalin-alcohol-acetic solution, mate- 

 rial is likely to sink promptly, and the pump is not necessary. 



For the younger stages, 3-5 /i is a good thickness; but for older 

 stages, after the necks of the archegonia have begun to curve, 10 or 

 even 15 n may be necessary if the egg, ventral canal cell, and neck 

 canal cells are to appear in one section (Fig. 80). 



For stages in the cutting off of the ventral canal cell and fertilization 

 of the egg, sections should not be thicker than 5 jj.. 



In Riccia nutans the direction of the axis of the archegonium at 

 every stage in the development must be known; otherwise, there will 

 be few good longitudinal sections. 



In forms like Porella and Scapania, the involucre covering the 

 archegonia is likely to hold a bubble of air, which will delay or even 

 prevent fixing. The best plan is to cut off the offending leaf with a pair 

 of slender-pointed scissors. Sometimes the air can be got out with an 

 air-pump. 



The sporophyte. — Sporophytes in early stages of development often 

 yield good preparations without very much trouble, but in the later 



