196L] 



BOTAXY OF THE GUAYANA HIGHLANDS — PART IV 2 I 



9 



Tyleria Gleason, Bull. Torrey Club 58:391. 1931. 



This interesting genus began its taxonomic history with the publication by 

 Gleason in 1931 of four species, all collected on Cerro Duida, viz. Tyleria 

 floribunda, T. grandiflora, T. linearis, and T. spathulata. Only the last has been 

 found elsewhere, being a prominent element of the cumbre flora of Cerro 

 Huachamacari. a sandstone mountain that lies to the north of and adjacent 

 to Cerro Duida, and on Serrania Paru, which lies some 150 km to the north. 



Our more recent visits to Cerro de la Xeblina. more than 300 km to the 

 south of Duida, have resulted in the discovery of four additional species. It is 

 not to be unexpected that further species of Tyleria will be found as the great 

 cerros of Xeblina and Duida and the immediately associated tabular mountains 

 as yet unvisited by botanists become more fully explored or initially visited. 



For those presently known, a curious relationship develops. The four newly 

 discovered species of Xeblina stand as morphologic and geographic analogues 

 of their Duida congeners ; the eight species thus stand as four mutually bracketed 

 and related pairs. 



Tyleria floribunda of Duida and T. spectabUis of Xeblina bear overwhelming 

 morphologic evidence of immediate affinity in habit. In form of leaf and in- 

 florescence they are exceedingly similar, differing largely in leaf apex and 

 margin and critical characters of the flower. T. floribunda, the largest tree 

 of the genus on Duida, is strikingly fastigiate. T. spectabUis, the largest tree 

 of the genus on Xeblina, is fastigiate-candelabriform. 



■ Tyleria pendula is certainly the analogue of T. spathulata, the inflorescence 

 of the first being pendulous and more nearly racemiform, and the leaves more 

 strongly petiolate and strongly aristate. The two occupy comparable ecological 

 niches in their respective geographical areas. 



Tyleria linearis of Duida and T. aristata of Xeblina are both wiry-stemmed 

 shrubs of closely similar habit, forming dense thickets along water-courses. 

 Their flowers are solitary and axillary; yet the leaves of T. linearis are, as the 

 name indicates, narrowly linear and sessile and quite devoid of an arista, while 

 the leaves of its Xeblina counterpart are elliptic, petiolate, and strongly aristate. 



The fourth pair, Tyleria grandiflora and T. tremuloielea, are less obvious 

 analogues, yet they are probably most nearly interrelated between themselves 

 than to other species of the genus. 



It is thus most interesting to postulate a parallel evolution of each of the 

 pairs from a common forerunner, or the segregation of one of each set directly 

 from its analogue. If such a postulatiou holds, and it is abundantly supported by 

 morphologic analogy, it is exceedingly difficult to give basis to genetic explana- 

 tion for the presence of the strong aristate leaf-tip that obtains absolutely for 

 the Xeblina series, and is absolutely absent from the Duida series. This char- 

 acter, certainly of genetic significance, cuts directly across otherwise clear lines 

 of affinity of the eight species so involved. An alternative explanation is the 

 evolution from two ancestors, with parallel adaptation to similar habitats. 



Key to the Species of Tyleria 



1. Inflorescence broadly compound-paniculate; flowers rose or pink; leaves coriaceous 

 or subcoriaceous, oblong-elliptic, normally exceeding 12 cm in length. 

 2. Leaves neither scarious-margined nor aristate (in material available) ; sepals 

 elliptic-lanceolate, not scarious-margined, acutish, at maturity 10-12 mm long ; 

 internodes of the branchlets 4-10 mm long ; leaf -scars transversely lunate or sub- 

 | reniform. ■ T. floribunda. 



