212 



Methods in Plant Histology 



In Riccia natans (Ricciocarpus natans) 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. Some- 

 times 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 later stages they are 

 frequently difficult to cut on account of 

 the secondary thickening of the capsule 

 wall and the stubborn exine of the ma- 

 ture spores. Great care must be taken to 

 get Riccia natans into paraffin without 

 shrinking, and the same thing may be 

 said of other forms which have such loose 

 tissue with large air cavities. Formerly, 

 we resorted to celloidin for stages like that 

 shown in Fig. 64. The gradual processes 

 already described have obviated the diffi- 

 culty, so that the student should be able 

 to get thin paraffin sections as free from distortion as were the 

 old celloidin sections. In Riccia natans it is even more difficult to 

 get median longitudinal sections of the sporophyte than of the arch- 

 egonium. Sections perpendicular to the groove, whether longitudinal 

 or transverse, are almost sure not to give median longitudinal sec- 

 tions 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 sections 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. 



FIG. 64. Riccia natans: young 

 sporophyte inclosed in the arche- 

 gonium; spore mother-cell stage 

 Delafleld's haematoxylin. All 

 the cells of the sporophyte, ex- 

 cept a single peripheral layer 

 (dotted in the figure) produce 

 spores. Celloidin section 30 M in 

 thickness. X 104. 



