CHAPTEK XYI. 

 THE STRUCTURE OF STEMS, 



Now that you have seen the various kinds of cells, 

 fibres, and ducts, and the tissues they form, the next 

 step is to discover how they are put together in the 

 construction of roots, stems, leaves, flowers, fruit, and 

 seeds. To do this we begin with the embryo, and 

 trace its development from germination to maturity. 



As the growing radicle, the chief part of the 

 embryo, from which roots, stem, and leaves proceed, 

 is itself stem, we will study the stem first. 



EXERCISE LXVI. 



Structure of Dicotyledonous Stems. First Year's 

 Growth. 



The monocotyledonous embryo starts with a sin- 

 gle lobe, turned a little to one side of the plumule, 

 while the dicotyledonous embryo develops two oppo- 

 site lobes, spreading away laterally from the plumule. 

 This is but the beginning of a series of differences in 

 structure, which these two classes of plants will pre- 

 sent as growth proceeds ; hence, they must be studied 

 separately. We begin with dicotyledons. 



You are familiar with the succession of external ap- 

 pearances presented by the growing plantlet. In Fig. 

 383 (Gray) you see the part which was the embryo 

 ending above, in the plumule, and below, in the root. 



