52 BULLETIN 791, U. S. DEPAKTMEFT OF AGRICULTURE. 



inch and the 12 to 24 inch soil layers there was available water 

 until about September 10. Because of the desiccation of the upper 

 soil layer during the first few days in August at least 90 per cent 

 of the vegetation ceased to function. While the rate of growth and 

 the period of maturity vary according to the character of the season, 

 the ruderal-weed cover generally requires only about six weeks in 

 which to complete growth and mature its seed crop. 



THE EFFECT OF DISTURBING FACTORS. 



The edaphic conditions of few of the deeper and older mountain 

 soils are so adverse as entirely to exclude the first-weed-stage plants. 

 But the density of the cover of the type in question is controlled 

 largely by the proportion of available moisture contained in the 

 soil during the germination period. Likewise the luxuriance of the 

 stand is determined chiefly by the available moisture content during 

 the growing season. 



Thorough' colonization of the ruderal-weed species for three or 

 more years, in the absence of the play of factors adverse to the un-~ 

 hampered development of the vegetation, generally calls into evi- 

 dence an admixture of the more aggressive, short-lived species of 

 the second- weed stage, hence marks the initial passing of the ruderal- 

 weed cover. On the other hand, the removal of a large proportion of 

 the soluble plant foods, and indeed of portions of the soil itself, 

 is followed by a distinct retrogression of the first-weed-stage vegeta- 

 tion. Where the destruction of the soil is continued to the extent 

 that the " holdfast " of the ruderal-weed type is destroyed, the pio- 

 neer stage of plant life — the algae and lichens — again appears over 

 the exposed rocks. 



PALATABILITY. 



With the exception of a very few plants, the ruderal-weed conso- 

 ciation affords little feed for stock. A large number of the plants are 

 cropped b}^ sheep at least to a limited extent, but they afford only a 

 small amount of forage. Cattle graze only a few species, and horses 

 consume practically none. The most palatable plant to sheep and 

 cattle is tansy mustard, the flowers, leafage, and the more tender 

 parts of the stem being devoured with unusual relish until the 

 plant reaches maturity; after that, like nearly all of the first-weed- 

 stage species, it is not grazed noticeably at all. Tansy mustard, how- 

 ever, occurs so scatteringly over the type as a whole that it is of 

 little value as a forage plant. Douglas knotweecl, the most abundant 

 of the ruderal-weed species, is grazed to a considerable extent by 

 sheep before the seed is disseminated, apparently with good results 

 to the animals. It probably furnisher more feed than all the rest of 

 the annual species combined. Cattle also graze Douglas knotweed. 



