Floristic Interchanges — Li 
181 
NORTHWARD MOVEMENT OF 
TROPICAL ELEMENTS 
In contrast to the southward migration of 
these temperate elements, the northward move- 
ment of tropical elements from the Philippines 
to Formosa is limited practically to the low- 
lands and particularly to the southern ex- 
treme of the island of Formosa. Favored by 
the gradual rising of temperature since the 
Pleistocene, these tropical, or southern, plants 
that find their way to Formosa are able to 
establish themselves there. 
Apparently many strand plants migrate by 
the help of ocean currents. In the case of 
Formosa, the main current is supplied by the 
Japan Stream from the south which passes 
along the island chain connecting the Philip- 
pines and Formosa. This chain of small islands 
also acts as a series of stepping stones for the 
migration of plants, especially from south to 
north. This northward migration of plants is 
also aided by wind. The prevalent monsoon 
during the summer and fall and the general 
track of typhoons rampant in the eastern Paci- 
fic during this same period of the year are also 
in a northeastern direction from the tropics, 
often leading from the Philippines to For- 
mosa. 
That these islands serve as an efficient mi- 
gration route of the northward movement of 
southern plants can be demonstrated by the 
presence of the following floral elements in 
Formosa: (1) species of wide tropical distri- 
bution, (2) species common to the Philip- 
pines and Formosa, (3) species common to 
the Philippines and the southernmost part of 
Formosa, and (4) species common to the 
Philippines and the islands of Lutao and 
Lanyu. 
In the first group are many paleotropic 
species that extend to the whole island of 
Formosa but more commonly only to the 
southern part of the island. Such common 
species are Dodanea viscosa (L.) Jacq., Croton 
cumingia Muell.-Arg., Litsea cuheha Pers., and 
Murrya paniculata (L.) Jack. Those that also 
occur in the northern part are mostly along 
the coastal lowlands. Their wide ranges are 
due apparently to their more aggressive na- 
ture and especially to their wider tolerance of 
temperature extremes. 
In the second group are species like Acacia 
confusa Merr., Ipomoea polymorpha R. & S., and 
Oreocnide trinervis Wedd., which occur widely 
in the Philippines and extend also to Formosa, 
especially along the coasts and lowlands, from 
south to north. The number of such species 
with wide ranges in Formosa is few, indicating 
the insignificance of such elements and pos- 
sibly also their recent and scattered arrival. 
There are many more species common to 
the Philippines and Formosa but confined in 
the latter island to the southern part. These 
species are sometimes limited only to the 
southernmost tip of the island — the Hun- 
chuen Peninsula and the small islands of Lu- 
tao and Lanyu. Such species as llligera luzon- 
ensis Merr., Schizostachyum diffusum Merr., 
Goniothalamus amuyon Merr., Aglaia elliptici- 
folia Merr., Maha huxi folia Pers., Guettarda 
speciosa L., Morinda citri folia L., Pemphis acidula 
Forst., etc., are often important elements in 
the local vegetation. That their occurrence is 
often limited to the seashore and coastal areas 
in Formosa distinctly indicates that they are 
relatively recent arrivals. 
The flora of the two small islands, Lutao 
and Lanyu, off the southeast coast of Formosa 
is much more intimately related to the south- 
ernmost part of Formosa than is generally 
understood (Li and Keng, 1950). The south- 
ernmost tip of Formosa manifests close floris- 
tic relationships with the small islands as well 
as with the Philippines in general. Its flora is 
quite distinct from that of northern Formosa. 
The fact that some species are present on these 
two small islands but absent in the southern- 
most part of Formosa is due apparently to 
human factors, as the mainland of Formosa 
has been more severely exploited. As a result, 
there are a number of Philippine species that 
are present on these islands but do not occur 
on the mainland of Formosa, such as Myris- 
tica cagayanensis Merr., Boerlagiodendron pecti- 
