228 



Methods in Plant Histology 



fixed in formalin alcohol. Safranin and anilin blue, with or without 

 a little orange, is a good combination. 



The Strobilus. E. arvense affords the most favorable material 

 for a study of the development of sporangia, since the strobilus con- 

 tains almost no silica and, even in its latest stages, is easily cut in 

 paraffin. In this species, the young strobili are recognizable in July, 

 the sporangia with sporogenous tissue are formed in August, and 



the divisions in the spore mother 

 cell occur in September. The 

 spores are not shed until the 

 following April. If you know a 

 patch of this species which 

 " fruits' 1 every year, dig up 

 the horizontal underground 

 stem in July. The tip of the 

 main axis is almost sure to be 

 a strobilus. Dissect away the 

 scale leaves and fix the strobilus 

 in chromo-acetic acid with a 

 little osmic acid. August and 

 September stages are easy to 

 recognize. If strobili are 

 brought into the laboratory in 

 December or January, they shed 

 their spores within a week. 



Strobili of other species, like 

 E. limosum and E. hiemale, con- 

 tain a large amount of silica and, 



consequently, only the younger stages cut well in paraffin. Hydro- 

 fluoric acid damages the cell contents more or less. In species like 

 these, all stages in the development are found in a single season. 



The Gametophytes. The spores of Equisetum germinate as soon 

 as they are shed. They retain their power of germination only a day 

 or two. Shake the spores from the strobili directly upon the soil. 

 Sow spores in an ordinary flower pot in the greenhouse, or use a glass 

 dish. In the latter case, break up pieces of flower pot for a bottom. 



m\T -. 



. & 



* 



.1 



FIG. 75. Equisetum arvense: photomi- 

 crograph of prothallia with antheridia. 

 X30. 



