1gIo] CURRENT LITERATURE 477 
different geological levels, Lower Cretaceous and Upper Jurassic, as proving 
their hypothesis that the pith of the Osmundaceae is of stelar origin and that 
the medullate forms are an ascending series, beginning with species which 
have a pith but no foliar gaps separating the collateral bundles, and ending 
with those in which the bundles are concentric and separated by widely patent 
gaps. So far as the testimony goes, which they have brought forward in their 
various memoirs, this hypothesis appears to bear a surprising resemblance to 
a confession of faith, since it is essentially the substance of things not seen but 
hoped for.—E. C. JEFFREY. 
Parasites and xeno-parasites.—The extensive investigations of CANNON™ 
upon the root habits of desert plants has added Orthocarpus purpurascens and 
two species of Krameria to the list of semiparasitic plants. Orthocarpus,» 
the first to be reported, is an annual of small size, united by its roots to a con- 
siderable variety of hosts, of which most are also annual in habit. Krameria 
canescens and K. parvifolia, two shrubby desert plants, were formerly taken to 
be autophytic, and the present study seems to show that they have not advanced 
far in parasitism. The seeds germinate readily irrespective of the presence or 
absence of the host plants, and the seedlings are capable of independent exist- 
ence for an indefinite period, and yet when associated with possible hosts the 
root tips of Krameria organize haustoria penetrating the roots of the host and 
forming therewith close vascular connections. Krameria parvifolia has been 
found on Parkinsonia microphylla only, but K. canescens has a variety of hosts, 
being most frequently associated with Covillea tridentata. These parasites 
show but few of the atrophies or alterations usually accompanying dependent 
nutrition. 
n making a study of the conditions which may have led to the develop- 
ment of parasitism, MAcDoucGat?9 2° has succeeded in uniting various plants, 
seasons. Various species of cactus were usually chosen as hosts, Echinocactus 
and the giant Cereus figuring most conspicuously in this r6le, while the ‘‘xeno- 
parasites” included the Mexican grape, agave, various cacti, and other succu- 
lent plants. After the union of the host and parasite was effected, there 
appeared in the development of the latter the atrophies and reductions charac- 
teristic of parasites. It appears that the relative acidity of the sap of the two 
plants has no part in determining their potentialities as parasite and host, but 
for the one to draw its nutrition from the other the latter must possess sap of 
a lower osmotic activity than the former, although not all plants thus related 
% MacDovucal, D. T., and Cannon, W. A., The conditions of parasitism in 
plants. Publication no. 129, Carnegie Tastitution ot Washington. I9fo. 
” CANNON, W. A., Parasitism of Orthocarpus purpurascens. Plant World 12: 
259-261. 1909 
2° MacDoveat, D. T., The making of parasites. Plant World 13:207-214. 1910. 
