INTRODUCED PLANTS OF A DESERT VALLEY 277 



gation canals, and the other side of the road is used. When this, 

 in turn, becomes bad the process is reversed. In this way some 

 weeds are enabled to maintain themselves by the wayside. 



Such of these plants as need a constant supply of water are by 

 reason of that requirement confined to the banks and margins of 

 the main canals. In these places occur some shrubs that are 

 entitled to be considered as weeds. On the banks of a newly 

 constructed canal there first appears a fringe of Pluchea sericea 

 and Baccharis glutinosus, and when these are removed there often 

 comes up a close grow^th of the slender cane-like stems of Salix ex- 

 igua. Mingled with the shrubs, or often occupying the entire banks 

 is a luxuriant growth of Physalis Wrightii and Amaranthus Pal- 

 meri. Very infrequently are to be found small colonies of Datura 

 discolor and Sesbania macrocarpa. The latter tends to spread into 

 the fields. While under favorable circumstances it may attain 

 the height of a dozen feet, the seedling plants begin to fruit 

 before they have grown to half that number of inches, so that 

 with such facilities for propagation it is likely to increase in abun- 

 dance, although its heavy seeds are not adapted to water trans- 

 portation. 



The shallow margins of all the older canals are grown up with 

 Scirpus paludosus, and as it is practically impossible to eradicate 

 it, constant labor is necessary in order to prevent it from check- 

 ing the flow of the stream. It does not flourish on those ditches 

 to which water is admitted only at stated intervals, but there its 

 place is taken by a rank growth of Echinochloa zelayensis and 

 Paspalum distichum, which is almost as troublesome. 



Altogether the worst weed of the Valley is Aster spinosus. It 

 grows in clumps of several slender and nearly leafless stems two 

 or three feet high, whenever there is sufficient moisture in the soil 

 to supply its demands. It is known to the farmers as "wild as- 

 paragus," not from any resemblance of its woody stems to the 

 succulent sprouts of the garden vegetable, but because, like that, 

 it has tough matted rhizomes, very resistant to the plow, which, 

 cut in pieces, originate new plants. It is said that when a field 

 becomes badlj^ infested it can be cleared only by allowing it to lie 

 unirrigated for a year — to return temporaril}^ to desert conditions. 



