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A TEXTBOOK OF THEORETICAL BOTANY 





such as the grasses, where it is a necessary consequence of the hollow 



internodes. 



5. Cortical Bimdles. These are leaf trace bundles, usually lateral ones, 

 which do not enter the ring but pursue an independent course through the 

 cortex. They may either enter the ring at a lower node, as in Begonia, or they 

 may unite with the cortical bundles of the next internode below, thus forming 

 an independent system connected to the ring only by nodal anastomoses, as 

 in Calycanthiis (Fig. 903). 



Conical bundle 



Fig. 903. — Calycanthus floridus. Transverse section 

 of stem showing one of the cortical bundles. 



6. '' Polystely." This term has been applied to the condition found in 

 some dicotyledonous types, i.e., Giinnera, Primula auricula and some related 

 species oi Primula, and in the Nymphaeaceae. The stem contains an irregular 

 network of connected bundles to which the leaf traces are joined. There 

 is no secondary thickening, the number of bundles increasing as the stems 

 thicken with age, as in Monocotyledons. The network is, however, much 

 more complex than in most of the latter group, though a similar structure is 

 characteristic of the Araceae. Each bundle has its own endodermis and 

 should therefore be a meristele, and the whole network has been called 

 an Atactostele, but it is probable that these meristeles are only concentric 

 amphiphloic bundles (Fig. 904). The suggestion has been made that this 

 peculiar condition has arisen secondarily from the reduced, central stele 

 characteristic of many water plants. When the stems subsequently increased 

 in size, by adoption of the land habit or otherwise, there was a need for more 

 vascular tissue. The capacity for secondary' thickening having been lost it is 

 supposed that the need could only be met by the branching of the single stele. 



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