i^ju uj killed the plant tissues. Our experiments at inducing male sterility with 

 Ethrel® failed to uncover any differential action on pollen and ovule formation such as 

 occur on wheat and other grasses (Rowell and Miller 1971). Our experiments with more 

 conventional techniques; namely, mass pollination, were more successful, but the results 

 were not definitive (see table 2, page 28) . 



The Tvidentatae do not appear to be self-sterile (table 2) . Seed was set in the 

 unopened control bags. We cannot make the case for self-compatibility point as strongly 

 as we would like to, however, because the pollination bags sift small amounts of pollen. 

 The bakery bags sifted no more pollen than commercial paper and woven cloth bags pre- 

 pared specifically for hybridization experiments. We (McArthur and Blauer, data on 

 file at the Shrub Sciences Laboratory, Provo, Utah) found only minimal amounts of 

 pollen on petroleum-coated microscope slides inside of the control bags. These tests 

 were made primarily with Atvirplex pollen but also with Artemisia pollen and unidentified 

 pollen. Pollen does not sift into the bags in quantity enough to produce seed sets of 

 up to 300 seeds per pollination bag (table 2). Apparently, therefore, selfing occurs. 



We believe, above and beyond the background of selfing, hybrid seed is also set. 

 Our 3 years' data (table 2) show a trend for higher interpopulation and intrapopulation 

 seed set than seed set in the control bags (selfing and sifting) . The differences are 

 not significant, perhaps because of the wide variations in seed set in the bags. The 

 environment within the bags was not natural. l\rhen the bags were opened, some enclosed 

 branches \<ere drier and some more moist than adjoining unbagged branches. 



CHARACTERISTICS OF THE GENUS CHRYSOTHAMNUS (RABBITBRUSH) 



This genus consists of much-branched subshrubs or shrubs up to 3 m high with 

 ascending to erect stems bearing alternate, narrow, deciduous leaves. The herbage 

 ranges from glabrous to densely tomentose and is commonly resinous and aromatic. 



Rabbitbrush flowers are borne in heads that in turn are arranged into cymes, 

 racemes, or panicles (fig. 35) . The heads usually contain from 5 to 15 yellow or 

 sometimes white disc flowers and are subtended by involucral bracts (fig. 36) arranged 

 into 5 more or less distinct vertical ranks. Each flower contains a pappus of abundant, 

 white, slender capillary bristles (fig. 37), a 5-toothed or cleft tubular to funnel- 

 form corolla, 5 stamens united by their anthers around the style, and a pistil with 



