299 



" Between the genera the contrast is naturally greater, and manifests 

 itself not only according to latitude, but also longitude. Thus, in the 

 torrid zone, the genus Paspalus has a decided preponderance in the New 

 World. Most of the genera, however, especially the larger, for example, 

 Panicum, Andropogon, Chloris, are every where nearly equal, those that 

 are peculiar being generally not at all numerous. The generic difference 

 between North America and the temperate regions of the European con- 

 tinent is very small. In North America, however, a greater number of 

 tropical forms appears. Between the two temperate zones also the distinc- 

 tion seems to be by no means considerable. Of 36 genera from the Cape, 

 30 occur in the temperate zone of the northern hemisphere, while, in other 

 families, southern Africa has many peculiar to itself. In the extra-tropical 

 part of New Holland the greater number of genera is found also in the 

 north (about 2-3ds) ; and this appears to be still more the case in the 

 southern parts of South America, as well as New Zealand. One of the 

 most extensively distributed genera is Poa. It is found almost over the 

 whole earth ; and, although it reaches its maximum in the temperate, has 

 also many species in the torrid zone. 



" What has been said of the decided influence of the degrees of latitude 

 on groups and genera, holds also of the habitus of vegetation in general. 

 The greatest differences between tropical and extra-tropical Grasses appear 

 to be the following: — 



" 1. The tropical Grasses acquire a much greater height, and occasionally 

 assume the appearance of trees. Some species of Bambusa are from 50 to 

 60 feet high. 



"2. The leaves of the tropical Grasses are broader, and approach more in 

 form to those of the other families of plants. Of this the genus Paspalus 

 affords many examples. 



" 3. Separate sexes are more frequent in the tropical Grasses. Zea, Sor- 

 ghum, Andropogon, Olyra, Anthistiria, Ischtemum, /Egilops, and many 

 other genera, which only occur in the torrid zone, and are there found in 

 perfection, are monoecious, or polygamous, Holcus is perhaps the only 

 extra-tropical genus with sepai'ate sexes. 



" 4. The flowers are softer, more downy, and elegant. 



" 5. The extra-tropical Grasses, on the contrary, far surpass the tropical 

 in respect of the number of individuals. That compact grassy turf, which, 

 especially in the colder parts of the temperate zones, in spring and summer, 

 composes the green meadows and pastures, is almost entirely wanting in the 

 torrid zone. The Grasses there do not grow crowded together, but, like 

 other plants, more dispersed. Even in the southern parts of Europe, the 

 assimilation to the warmer regions, in this respect, is by no means incon- 

 siderable. Arundo donax, by its height, reminds us of the Bamboo ; 



