INTRODUCTION 33 



and this is important in distinguishing different 

 types. 



In the stem the structures are different in the two 

 groups Dicotyledons and Monocotyledons, as has 

 been pointed out in the Introductory Volume. The 

 former were called exogens, the latter endogens. In 

 the Monocotyledons (see Fig. g) there is no con- 

 tinuous ring of wood and bast, but they are scattered 

 in the ground tissue. There is not usually a distinct 

 pith, and the cortex is thin. 



In Dicotyledons herbaceous plants have a less 

 well-developed woody layer (see Fig. 10), though it 

 resembles in the main that of young trees. The 

 latter have the power to increase the thickness of the 

 wood by secondary tissues, supplied by the activity 

 of the cambium, whose initial cells divide up. The 

 secondary tissue resembles the primary wood, etc., 

 but the vessels and tracheids in the former have 

 bordered pits and reticulate markings, and are not 

 thickened with spiral or annular rings. 



The modifications in structure of the stem are 

 correlated with the amount and mode of branching, 

 the need for support and the mode of increase in 

 thickness. The tissues themselves are built up with 

 regard to the carriage of water with salts in solu- 

 tion, or nutrient substances up or down the stem, 

 and the direction and distribution of these is variously 

 achieved by different contrivances for their effective 

 circulation. 



Leaf structure, like that of stem structure, varies 



VOL. III. 3 



