CHAPTER XXVII 



SPERMATOPHYTES 



ANGIOSPERMS 



This immense group demands knowledge and skill in the whole field 

 of histological technique, for embryo sacs are so delicate that they are 

 as difficult as the free nuclear stage in the female gametophyte of a 

 gymnosperm, while the peach stone needs a petrotome rather than a 

 microtome. Between these extremes there is everything imaginable in 

 structure, chemical composition, and consistency. 



Some hints will be given, but the student will gradually learn what 

 should be cut freehand and what should be imbedded, 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 ; and what fixing agents are likely to 

 give the best results with various kinds of material. 



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; but it should be re- 

 membered that the range of structures which can be imbedded in 

 paraffin can be increased by the use of hydrofluoric acid and improve- 

 ments in the paraffin method. 



The stem.— The vascular cylinder of the angiosperms is either an 

 endarch siphonostele, or a polystele derived from it. 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 di- 

 ameter, cut freehand sections and fix them in formalin acetic alcohol 

 for 24 hours. The sections may be left here for months. Transfer to 

 50 per cent alcohol and leave them here until all chlorophyll is ex- 

 tracted. Probably 24 hours will be sufficient. Then stain in safranin 

 and light green, or safranin and Delafield's haematoxylin. Such sec- 

 tions will show both primary and secondary structures in both stele 

 and cortex. Higher up, there will still be secondary structures in the 

 stele, but none in the cortex; and still higher up will be found the 

 origin of interfascicular cambium. All of these can be cut without im- 

 bedding, but the earlier stages showing the differentiation of protoxy- 



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