The largest of these tracts is Imperial Valley, a body of rich alluvial soil 

 situated in the southwestern part of the Colorado Desert, beneath the level of 

 the sea. In 1902 water from the Colorado River was carried on to these lands 

 sufficient to irrigate 300,000 acres, and these are now under intensive cultivation. 

 In 1912-1913 a survey of the flora of this valley showed that the majority of the 

 weeds common in the older settlements, had not yet arrived, and that most of 

 those which had made their appearance were, as yet, infrequent. It is probable, 

 however, that an investigation made at the present time would show a great 

 increase in the number and abundance of exotic weeds. Two other facts in rela- 

 tion to the weed flora were brought out by this survey; namely, that not an im- 

 migrant plant was found on the open desert beyond the limit of irrigation; and 

 that the weeds most troublesome to the farmer were plants indigenous in the 

 overflowed lands of the delta, carried by the irrigation system onto the fields, 

 where the}' flourished with great luxuriance. Elsewhere in the deserts, where local 

 wells are the source of irrigating water, the surrounding native plants are not 

 likely to be attracted by cultivation; but in time the farmer will have to contend 

 with many of the common pests of agriculture, while the feral hills and plains 

 will remain largely immune from their invasion. 



THE MOUNTAINS 



The mountain area is the least extensive of the three under which our region 

 is considered. It is exceedingly rugged, its ascents abrupt, and in places pre- 

 cipitous; several of the summits exceed 10,000 feet in altitude, and the loftiest at- 

 tains 11,725 feet. Above 5,000 feet they are mostly covered with an open forest of 

 pines and other conifers. Neither the soil, nor, in the higher parts, the climate, 

 is favorable to cultivation, but below the 4,000 feet contour there are limited tracts 

 where it succeeds. Consequently the immigrant flora is scanty, both in species 

 and in individuals. Some increase is to be expected, since good automobile roads 

 now render access easy to many parts of the mountains, drawing thousands to 

 them for their summer vacations. 



THE CISMONTANE REGION 



The cismontane is a region of fertile soils, where an extensive and varied agri- 

 culture is carried on, mostly by irrigation. It is filled with cities and towns, with 

 their accompanying industries; and in all parts it is well served by railways, three 

 of them transcontinental. It was also the earliest settled part of the State, and the 

 site of the first missions and of the earliest pueblos. 



These conditions naturally result in the presence of an abundance and diversity 

 of introduced plants. They may be considered in two groups, which are determined 

 by their growth-requirements. The first includes plants whose long period of 

 growth needs a continuous seasonal supply of water, and to which, in most cases, 

 a rich soil is essential. These requisites are to be found in the farms, the orchards, 

 the gardens and lawns, and other cultivated and irrigated lands, or in (he infre- 

 quent small tracts naturally moist. Here grow the common weeds of cultivation, 

 with a few of more restricted range: and here are now appearing other cosmo- 

 politans, often as yet mere casuals. To this group belong the deep-rooted annuals, 

 and all the perennials save one. 



Most plants belonging in the second group, while found in cultivated grounds, 

 are also able to contend with the native plants of the feral mesas and hills. To do 

 so they must share the life adaptations of the indigenous vegetation with which 

 they are brought into competition. This consists mostly of annual herbs, which 

 complete their cycle of existence during the few rainy months, or of perennials, 

 mostly shrubby or suffruticose, able, by reason of special adaptations, to survive 

 the long dry season. A single introduced perennial, horehound, is of this class; 

 the others are quick-growing annuals, mostly grasses. Here belong a number of 

 Mediterranean plants, dating from the mission era, now widely spread over the 

 state, and a few pestiferous bromes, recently introduced, but disseminating them- 

 selves with great rapidity. The number of species is not great, but in individuals, 

 and in the extent of ground which these weeds occupy, and often monopolize, they 

 exceed all the others combined, and little or nothing can be done to combat them. 

 The early immigrants possessed qualities which made them a valuable addition to 

 the plant population, but the later-comers are entirely worthless; but good and 

 bad alike have greatly modified the native vegetation, and in places replaced it. 

 In very many thev are the prominent feature of the plant covering, and impart to 

 the landscape a foreign aspect. It may well be that in no long time extensive 

 bodies of an unmixed native flora will be found only on the arid deserts or the 

 higher mountains. 



BEHAVIOR OF THE IMMIGRANT FLORA 



The intrusion of man has thus disturbed the equilibrium attained by the long 

 interaction of natural causes. Not only have his various activities been directly 

 or indirectly destructive to the native vegetation, but he has introduced new, and 

 often vigorous, competitors in the struggle for existence. Were he now to with- 

 draw, the forces of nature, again free from his interference, would tend to the re- 

 storation of former conditions. In time new forests would replace those the wood- 

 man has felled, and the old chanarral would repossess the cleared hills. These 

 tracts have been practically unaffected by the introduced vegetation; but not so 

 the open mesas and slopes and the naturally damp meadows. Here, in the farms. 



