64 REPORTS OF INVESTIGATIONS AND PROJECTS. 



their roots occupy horizontal layers at different depths. Competition there- 

 fore takes place between species which spread out their roots at the same 

 depth, and in this regard the luxuriant growth of annuals characteristic of 

 the moist midwinter and of the moist midsummer may come into direct 

 competition with the perennials which send their roots to the same depths. 



The type of root-system, or rather the habit of the root-system, of a species 

 is a determining factor in local distribution of desert plants to a much 

 greater extent than it may be in the plants of moist regions. Some species, 

 such as the creosote-bush (Covillea), which have a general habit and great 

 plasticity, are capable of living in a wide variety of habitats, while such forms 

 as the sahuaro (Carnegiea) which require both anchorage and great super- 

 ficial extension of the root-systems, are necessarily confined to certain narrow 

 types of habitats, although disseminated over a wide extent of territory. 



Parasitism in Desert Plants (by Dr. W. A. Cannon) : 



The critical examination of the root-systems of many hundreds of plants 

 indigenous to the Tucson region brought to light many facts of interest in 

 connection with the prevalence and origination of parasitism. Orthocarpiis 

 purpurascens, one of the annual Scrophulariacese, was found to fasten to the 

 roots of nearly twenty species of plants, all but two of which are annuals. 

 Several other members of the family of Orthocarpus have long been known 

 to be parasitic, but information as to the habits of this plant was lacking. 



Two species of Krameria, K. canescens and K. parvifolia, of the family of 

 Krameriacese, were found to be parasitic on a number of woody perennials, 

 inclusive of trees, shrubs, and cacti, and the hosts are of both the spinose 

 and succulent types. The parasitism of Krameria is very destructive to its 

 hosts, in consequence of which no unions of any great age could be found. 

 The minute structure of the haustoria, or penetrative and absorbent struc- 

 tures, of the parasite have been followed and display many features of 

 interest which will be described in a paper now in course of preparation. 

 The establishment of the parasitic relation was followed experimentally, 

 using Parkinsonia aculeata as a host. 



A South American species of Krameria has been an article of commerce 

 for a long period, the roots yielding a dye used in coloring wines, but no 

 mention seems to have been made of any parasitism of this plant. The at- 

 tachment to the host is made by long, thin roots, which might easily be de- 

 stroyed in digging, and it is quite probable that a careful examination of 

 the South American species would show that it also fastens to the roots of 

 neighboring species. 



Inheritance of Structural Characters in Hybrids (by Dr. W. A. Cannon) : 



Dr. Cannon has completed the proofs of his paper (Publication No. 117) 

 on the inheritance of structural characters in hybrids, and has made some 

 further tests which confirm the conclusions set forth in the paper. In the 



