338 GENEBAL BOTANY 



The herbaceous dicotyledons resemble the herbaceous ferns 

 in having a wide storing cortex, a large pith, and a narrow vas- 

 cular cylinder. They differ from the ferns and are like the woody 

 dicotyledons in the collateral structure of the phloem and xylem 

 and in the nature of the tissue elements, which are essentially 

 the same in all dicotyledonous stems. 



The storage system of cells in the dicotyledons, particularly 

 in trees and shrubs, is for the first time amply provided for by 

 large and small wood rays and by wood parenchyma abundantly 

 distributed throughout the primary and secondary wood. In 

 living pteridophytes the wood rays are lacking, and gymnosperms 

 have neither the rays nor the wood parenchyma so largely de- 

 veloped as in the woody dicotyledons. (Compare the figures 

 illustrating the anatomy of Adiantum, alder, and spruce.) This 

 highly developed storage system of the woody dicotyledons com- 

 pensates for the small size of the pith and cortex, which serve 

 the storage function for a short time only in these plants, since 

 the death of the cortex and pith in old dicotyledonous stems 

 relegates the entire storage function to the wood rays and the 

 parenchyma of the vascular cylinder. 



The conducting cells are the familiar ducts which constitute 

 long tubes for the rapid transfer of water necessitated by the 

 immense leafage of the broad-leaved dicotyledons. 



The average length of the tracheids which compose the water- 

 conducting elements of gymnosperms is from two to four milli- 

 meters, while that of the ducts in dicotyledons ranges from a 

 few centimeters to several feet in length. The ducts, therefore, 

 offer much less resistance to the rapid flow of water up the tree 

 trunk in a dicotyledon than the tracheids do in a spruce or other 

 gymnosperm (compare Figs. 53 and 193). 



The great differentiation in kind and arrangement of tissues 

 in the stems of dicotyledons is also a distinctive feature in these 

 plants, since ducts of various kinds and sizes, strengthening fibers 

 and tracheids, wood rays and storage parenchyma, are all adapted, 

 in them as in no other plants, to the proper performance of their 

 respective functions. This elaborate differentiation of tissues 

 culminates in the woody-stemmed trees and shrubs. 



