16 BOTANICAL GAZETTE [july 



The later development of the axillary bud is exactly the same as 



that of the main stem, with this exception: while in the stem the 



transition from seedling structure to the mature type is very 



gradual, the transition in the branch is much more rapid, being 



completed in about 6-8 internodes. After having once been 



attained, the mature type recurs constantly in each succeeding 



internode. 



Roots 



In the old seedling there are to be found four types of roots, 

 each having its characteristic structure. These are the primary, 

 secondary, and adventitious roots, and the branches of the latter. 



The primary root is diarch, maintaining this type of symmetry 

 even when mature. Its growing point is of the "Helianthus" 

 type, having distinct plerome and periblem initial groups, while 

 the calyptrogen and dermatogen have a common group of initials. 

 As the root matures, a cambium develops in a ring, broken opposite 

 the two protoxylem poles, forming two crescents of secondary 

 vascular tissue. The stele is surrounded by a sharply defined thin- 

 walled endodermis with Casparian dots. The cortex and epidermis 

 correspond to those of the adventitious roots of the mature plant, 

 sufficiently described by Holm. He, however, wrongly calls these 

 roots "secondary" (5, p. 319). 



The true secondary roots, that is the branches from the primary 

 root, possess the same type of growing point as the primary root. 

 The symmetry varies with the age, the younger parts of these roots 

 being usually diarch, and becoming later tetrarch or pentarch. 

 The general structure of these mature secondary roots is that of 

 the adventitious roots. The adventitious roots, formed at the base 

 of the seedling, and the branches of these adventitious roots, are 

 exactly like those described by Holm for the mature plant. These 

 also possess the "Helianthus" type of growing point. 



Abnormalities in the internal structure 



One very striking feature about the seedlings of Dianthera 

 americana is their extreme variabilitv. Two seedliners of the 



same 



same number of leaves developed, may 



very great differences in the degree of differentiation of vascular 

 tissue, cambium, endodermis, etc. For instance, a seedling of the 



