170 GENUS CHRYSOTHAMNUS. 



two species that are the most polymorphous in other regards, namely, viscidiflorus and 

 nauseosus. A considerable number of each of these, several hundred in all, were examined 

 in the field. The difference between the species was very marked. In all of the five sub- 

 species examined of the former, the main root divides near the surface of the soil, the 

 branches spreading out at a moderate depth and influenced in their direction by the 

 nature of the stony soil in which the plants grow. In the ten subspecies examined of 

 nauseosus, the root usually continues downward to a considerable depth as a well-defined 

 tap-root, emitting only secondary lateral rootlets. This species grows in better and 

 deeper soil than viscidiflorus. Along the edges of its habitat, where the soil is often shal- 

 lower or quite stony, the root habit may simulate that of viscidiflorus, but it is believed 

 that the two types are different in their origin. A striking peculiarity of the root is 

 common in C. nauseosus hololeucus. In this form, which inhabits dry sandy stream-ways 

 and fans, the large root is often twisted upon itself and easily breaks up into strands like 

 those of a rope. The cause of this is not known, but it is suggested that such roots are 

 really composite and have been formed by the intertwining of the single roots of several 

 plants. At any rate, such a possibility must be taken into account when making studies 

 in individual variation. 



Pubescence. — Two very distinct types of pubescence are encountered. Although char- 

 acters of the pubescence are generally untrustworthy and are easily modified by changes 

 in the environment, in this genus the two types are so unlike, the absence of intergrading 

 stages is so marked, and their appearance runs so nearly parallel with other characters, 

 that pubescence furnishes a safe guide even for sectional distinctions. This applies, 

 however, only to the presence or absence of a peculiar pannose tomentum. The number 

 and the distribution of ordinary hairs furnish criteria as unreliable here as elsewhere. 



The pannose tomentum referred to occurs in all of the Nauseosi but nowhere else in the 

 genus. It consists of a dense coating of long, weak hairs, which have become more or 

 less infiltrated with a resinous exudation from the twigs. The inner portion of this 

 mass closely invests the twigs like a tight-fitting coat. The outer ends of the trichomes 

 may be either loose, thus presenting a light, fluffy appearance, or they may be closely 

 impressed with the inner layers. In the latter case, the surface is often so smooth that 

 the woolly layer is mistaken for the bark itself, until it is scraped up by a knife-edge or 

 scalpel. This has led even careful botanists wrongly to describe certain forms as glabrous 

 or nearly so and thus to mask their true relationships. For example. Gray described 

 the branchlets of Linosyris bigelovi as with scarcely a perceptible pubescence; the same 

 author gives "greenish and minutely canescent" for the branches of Bigelovia juncea, 

 earlier described by Greene as cinereous ; Greene has described the twigs of C. pinifolius, 

 C. virens, C. laetevirens, and others as glabrous or nearly so; Nuttall, in characterizing 

 C. speciosus, made no mention of the pubescence of the stem, but at the same place he 

 proposed a variety albicaulis based only upon the dense and white tomentum of the stems 

 and suggested that it might be a distinct species. The types of all of these have been 

 examined and found to have twigs clothed with a dense felt-like tomentum. For this 

 and for other reasons they are now classified as subspecies of C. nauseosus. 



Aside from its presence or absence, the tomentum does not afford specific characters. 

 Its duration, amount, color, and extent are all too variable to indicate more than varietal 

 or subspecific rank. It is certain, however, that some of these qualities are heritable, at 

 least for a few generations. Certain subspecies of C. nauseosus, differing among them- 

 selves in the color and extent of the tomentum, come "true to seed" in these respects 

 when grown under uniform conditions in the Botanical Garden of the University of Cali- 

 fornia. The same holds for transplants made by transferring the roots of these sub- 

 species. While only extremes were used in these experiments, all intermediate forms are 

 found in the field (although not always in the same locality), so that such characters are 

 difficult to apply in the identification of described groups. 



