[Vol. 9 



240 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



in this differentiation between radical and cauline leaves. Have 

 these basal leaves been developed from amplexicaul stem-leaves 

 or has the change been in the other direction? Judging by analogy 

 with Caulanthus, in which genus the problem does not seem par- 

 ticularly difficult, it is supposed that the original radical leaf w T as 

 entire and amplexicaul and that it has given rise to the petioled, 

 lobed leaf independently from time to time. There seems to be 

 a constant tendency for entire leaves to become lyrate-lobed or 

 pinnatifid within the Cruciferae in general, although an advance 

 into xeropl lytic conditions seems frequently to be accompanied 

 by the reverse order. 



Trichomes. — Nine species of Thely podium are entirely glabrous 

 and the other five vary from glabrous or nearly glabrous to 

 quite densely pilose near the base. The trichomas are usu- 

 ally long and always unicellular and unbranched. No specific 

 specialization of trichome structure has been noticed in this 

 genus. The plants are ordinarily more or less glaucous as well 

 as glabrous. 



Those two species, T. eucosum and T. laciniatuni, which have 

 been considered primitive are glabrous, and because of lack of 

 evidence to the contrary it is assumed that the glabrous condition 

 gave rise to the pubescent one. The change is apparently not a 

 difficult one since in the same species glabrous and pubescent in- 

 dividuals occur. It is of interest to note that the trichomes when 

 they are present, appear only near the base of the plant. This 

 correlates in a significant way with the theory of the origin of rad- 

 ical from stem-leaves as will be discussed under Caulanthus. 



Duration. — All but three species of this genus are evidently 

 biennial, and of these three two are usually biennial but may 

 persist for several years, as T. sagittal urn and T. ovalifolium. 

 One only is definitely perennial as may be seen from the numer- 

 ous remains of former leaf-bases that clothe the caudex in T. 

 flexuosum. A definite assumption of the perennial habit doubt- 

 less represents a specialized condition. With this perennial habit 



is closely correlated the differentiation between stem and radical 

 leaves in which the latter form a definite rosette. The otherwise 

 more primitive species of the genus are biennial, and the peren- 

 nial or partially perennial species are the more specialized ones. 

 It is therefore assumed that the biennial preceded the perennial 

 habit in Thely podium. 



The generic limits of Thely podium. — As stated elsewhere, 

 many species have been referred to this genus that are not now 



