CHAPTER XXV 



SPERMATOPHYTES 



ANGIOSPERMS 



This group is so large, and its structures are so varied and complex, 

 that great care must be taken in the selection of material for sections. 

 With experience, one will gradually learn what stages in floral 

 development, what stages in the development of the embryo-sac, 

 or what stages in spermatogenesis are likely to be correlated with 

 easily recognized field characters. 



The Vegetative Structures. In stems, roots, and leaves the 

 more delicate structures should be imbedded in paraffin and the more 

 rigid structures should be cut without imbedding at all. 



The stem. Throughout the angiosperms, the vascular cylinder 

 is an endarch siphonostele. For a study of the development of the 

 stem, the common geranium (Pelargonium) may be recommended. 

 Near the base of a fresh stem, about 1 cm. in diameter, cut freehand 

 sections and fix them in 35 per cent alcohol for 10 to 20 minutes; 

 transfer to 70 per cent alcohol to extract the chlorophyll, and then 

 stain in safranin and light green. Such sections will show both 

 primary and secondary structures in the stele and in the cortex. 

 Higher up, there will be secondary structures only in the stele; and 

 still higher up will be found the origin of interfascicular cambium. 

 All these can be cut without imbedding, but the earlier stages show- 

 ing the differentiation of protoxylem, metaxylem, and the origin of 

 secondary xylem are too soft for successful freehand section?. Fix 

 in formalin alcohol and imbed in paraffin. 



For a study of woody stems, Tilia americana (basswood) is good, 

 and shoots from 5 to 10 mm. in diameter are easy to cut. Very hard 

 stems like Hicoria (hickory) and Quercus (oak) must be boiled and 

 treated with hydro-fluoric acid, if you expect to cut shoots more than 

 5 to 7 mm. in diameter. However, with a good sharp knife and a 

 rigid microtome much larger sections can be cut without resorting 



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