222 BOTANICAL GAZETTE [SEPTEMBER 
heterotricha and Collinsia Torreyi, which have entered through the 
same gate to share the anomalous position of the oaks. 
In the Colorado Desert the Yucca zone is replaced by a well- 
defined zone of Agave desertt, which, in an almost continuous 
belt between the altitudes of 2,500 and 4,000 * (760-1,220™), 
stretches along the desert slopes of the mountains from San 
Jacinto Mountain to and beyond the Mexican boundary. The 
plants of that zone, with inconsiderable exceptions, are quite 
distinct from those of the corresponding Yucca zone of the 
Mojave Desert, although it has the same altitudinal limits, and 
other physiographical conditions. 
Above this Agave zone there is a little known Pifion zone. 
It begins southeast of San Jacinto and extends to El Toro 
Mountain, above Toros, its upper limit being about 5,000“ 
{1,525™) above sea level. Possibly, in the little-explored 
mountains between El Toro and the Mexican line, other traces 
of this zone may connect it with the extensive nut-pine forests 
of the peninsula. At its upper end this belt is mainly composed 
of Pinos monophylla, while on El Toro, P. Parryana has sup- 
planted it.%3 
23 These two species appear either to coalesce, or to hybridize, in this belt. 
Specimens may be found in which there are, on the same twig, sheaths containing two, 
three, or four leaves. 
[Zo be concluded.| 
