ECOLOGICAL CHARACTERISTICS OF MARIANA ISLANDS. 15 
bush of the uplands without a transitional zone (plate 4, a and B). Two types are 
characteristic of the savannas of the Mariana Islands, as in Polynesia; they are the 
sword-grass, Xipheagrostis, and the somewhat xerophytic fern Gleichenia. Typi- 
cally there are no trees, but in the southwestern region especially the iron-wood, 
Casuarina, grows in some abundance, and sometimes beach shrubs like Lobelia 
casually establish themselves. The soil of the savannas is a red clay, very smooth 
and slippery during the rains and difficult to traverse on foot where the grasses are 
sparse. Such barren uplands occupy much of the territory in the southern half of 
Guam and constitute impassable barriers to the colonial interchange of snails dwell- 
ing in forested areas on their opposite sides; but much deforestation has been brought 
about, and undoubtedly the size and extent of the savanna barriers have increased 
during recent centuries. 
The strand itself is relatively dry and sparsely inflorated. Lobelia, Tourne- 
fortia, Casuarina, with occasional Paritt and Cocos, constitute the higher growth, 
while /pomea and trailing legumes spread over the sandy ground (plate 5 a). The 
inner beach has a more ample growth of trees, including Barringtonta, Terminalis, 
Paritt, Artocarpus calophyllum, Morinda, and ficoids such as Ochrosia; although it 
is not listed by Safford, Pandanus often occurs among these associates. Among 
the shrubs of the inner beach are Acacia, Ximenia, and Anona. This type of 
assemblage often attains considerable density, and serves as an abode of Partule 
(plate 5 B). 
Village environs are distinctly unfavorable by virtue of their open character; 
their plants are mainly introduced ornamental species, or are native types trans- 
planted from their natural environments elsewhere. The abandoned clearings near 
former settlements, however, are frequently reoccupied by Partule from the 
forests, especially in northern Guam, when the new growth attains the proportions 
of a thicket (see plate6 8). Various Leguminosz are joined by shrubs such as the 
“lemoncito” (Triphasia) and by the breadfruit, Pandanus, Pariti, and the true 
Hibiscus. The main points of difference from the forest are the absence of trees of 
high growth and the scarcity of cycads. : 
The cliffs bear a limited series of plants upon their abrupt faces, which are 
sometimes populated from adjoining forests upon the plateau; this association is so 
limited as to require only a formal citation. 
The flora of the river borders naturally varies greatly. Near the mouth of a 
stream like the Pago River or the Ylig River, the strand types occur together with 
other characteristic plants like the nipa palm. Further inland the hillsides may be 
open, and the savanna plants will come down very nearly to the river’s edge. Or 
it may be that the valley bottom is covered by typical forests and thickets, in which 
case the snails will be quite as much at home as in the dense growth of a plateau far 
distant from a large stream. 
The forests of low and high growth are the true homes of the Partule (plate 
6 a), although the sparser thickets at their borders and in the reoccupied clearings 
often serve nearly as well, provided only that the rainfall and soil are favorable. 
It would be impossible to find a more comprehensive and complete description of 
