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METHODS IN PLANT HISTOLOGY 



sections as free from distortion as were the old eelloidin sections. But 

 even with well-fixed material great care must be taken not to let the 

 paraffin get too hot. Remember that in most paraffin ovens the tem- 

 perature is different in different parts 

 of the oven. Do not let the tempera- 

 ture of the paraffin go above 53° C, 

 and, preferably, not above 52° C. In 

 Riccia natans it is even more difficult to 

 get median longitudinal sections of the 

 sporophyte than of the archegonium. 

 Sections perpendicular to the groove, 

 whether longitudinal or transverse, are 

 almost sure not to give median longi- 

 tudinal sections of the sporophyte, and 

 these are the sections the beginner is 

 sure to cut. Examine the material and 

 note very exactly the orientation of the 

 sporophyte; then, for fixing, cut out sec- 

 tions about 2 mm. thick, taking these 

 sections in such a plane that paraffin 

 sections parallel to the thick section will 

 give the desired median longitudinal 

 sections of the sporophyte. 



In forms like Pellia and Aneura, it 

 is desirable to show the sporophyte still 

 inclosed in the calyptra (Fig. 82). For 

 such sections, we should recommend fix- 

 ing in formalin acetic alcohol. 



For cytological studies, the calyptra 

 must be removed and a thin slab should 

 be cut from opposite sides of the cap- 

 sule to facilitate fixing and infiltration. 

 Chromo-acetic acid, with the addition of 

 a little osmic acid, is best for fixing. In Pellia and Conocephalus the 

 spores are very large and have a rather thin wall. Both these genera 

 show a peculiar intrasporal development of the gametophyte, i.e., the 

 gametophyte develops to a considerable extent before it ruptures the 

 spore wall and before it is shed from the capsule (Fig. 83). Mitotic 



Fig. 82. — Pellia epiphylla: sporo- 

 phyte nearly mature but still inclosed 

 within the archegonium. X40. From 

 Chamberlain's Elements of Plant Science 

 (McGraw-Hill Book Co., New York). 



