26 BOTANICAL GAZETTE [jULY 



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Rhexia virginica^ therefore, exhibits a very singular type of vegeta- 

 tive propagation, that is, one restricted entirely to the development 

 of root-shoots. 



THE INTERNAL STRUCTURE OF THE VEGETATIVE ORGANS 



The roots 



The secondary and lateral roots and the slender portions (apical 

 and basal) of the tuberous roots have the same structure, modified 

 only by age. In the tubers, on the other hand, there is a very different 

 structure, since they remain active for a longer time and are the seat 

 of a continuous development of aerial shoots, vegetative or floral. 

 In the capillary lateral roots the primitive root structure remains 

 unchanged, since they persist only one season. In the slender second- 

 ary and stronger lateral roots, which develop from the tuberous 

 portion, the structure becomes changed at an early stage on account 

 of the development of cambial strata from the conjunctive tissue on the 

 inner face of the leptome strands, resulting in an increase in thick- 

 ness of the stele and a gradual obliteration of the primitive structure. 

 These roots may persist and remain active, therefore, for a longer 

 time than do the capillary roots. However, the increase in thickness 

 of the stele does not always cause a rupture of the surrounding tissues, 



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accompanied by secondary formation (cork and secondary cortex); 

 the endodermis, cortex, exodermis, and epidermis often remain 

 entire for some time. As soon as the development of cork com- 

 mences (in Rhexia from the pericambium), the peripheral tissue 

 necessarily becomes broken, though without being thrown off. 



A much greater increase in thickness is seen, of course, in the 

 tuberous roots. In them, even during the very first period of their 

 growth, the activity of the pericambium is so rapid that the peripheral 

 tissues from endodermis to epidermis are thrown off completely, and 

 are replaced by strata of cork and secondary cortex originating from 

 the pericambium. Hence the structure of the tuberous roots shows 

 a marked deviation from that of the other roots. In jig. lo there are 

 shown the several layers of cork (P), the broad parenchyma of second- 

 ary cortex (C), and a continuous circle of cambium strata surround- 

 ing a broad cylinder of thin -walled parenchyma in which the long 

 but very narrow rays of mestome radiate from the primitive root-stele. 



