Chromosome Homology in Dendrohium — Kamemoto, Shindo, and Kosaki 
113 
Fig. 35. Diagrammatic representation of chromosome homology in hybrids of Dendrohium. The hybrid 
investigated was a monosomic. 
fertility. On the other hand, tetraploidy in intra- 
sectional hybrids results in reduced fertility due 
to the homologous parental genomes forming 
multivalents. 
D. superbiens was originally given species 
status but now it is generally recognized as a 
natural hybrid between species from the sections 
Phalaenanthe and Ceratobium (Holttum, 1957). 
The intersectional hybrid, D. phalaenopsis x D. 
undulatum most closely resemble D. superbiens 
in external morphology, but the large "marker” 
chromosome of D. undulatum was conspicu- 
ously absent in the particular plant examined. 
Further studies involving several individual 
plants collected from their natural habitat should 
clarify the cytological aspects of this natural 
hybrid. 
It appears that the taxonomic gap between 
Ceratobium and Latourea is much wider than 
that between Ceratobium and Phalaenanthe 
(Fig. 35), for the Ceratobium-Latourea hybrid 
exhibited an average of 10.8 bivalents per PMC, 
while the Ceratobium-Phalaenanthe hybrids av- 
eraged between 18.9 to 15.8. The gap between 
Phalaenanthe and Latourea is still greater as 
indicated by the very weak homology of parental 
genomes forming an average of only 1.8 biva- 
lents per PMC. 
The separation of species into the three 
groups, Ceratobium, Phalaenanthe, and Latourea 
appears to be valid on the basis of external 
morphology, cytology, or crossability. Cerato- 
bium and Phalaenanthe are phylogenetically 
much more closely related to each other than 
they are to Latourea. Also, it might be concluded 
that Latourea is more closely related to Cera- 
tobium than to Phalaenanthe. If evolution of 
these groups occurred in a sequential manner, 
then it is logical to assume that divergence 
proceeded from Latourea to Ceratobium to 
Phalaenanthe. 
Considerable differences in taxonomy and 
phylogeny might be noted for the Dendrobium 
genus and the Vanda alliance. Holttum (1957) 
has pointed out that: 
The flower-form throughout the tribe (Dendrobium) 
is remarkably constant; there is much greater varia- 
tion in vegetative characters. This is an interesting 
contrast to the Vanda-Arachnis tribe, in which floral 
form is very varied and vegetative form much less so. 
