210 THK PI.ANT WORI.D 



Of the remaining vegetation, the most characteristic forms near the 

 Laboratory are the ocotilla {Fouquieria splendejis) , the paloverde (^Parkin- 

 sonia microphylla) , species of cat's-claw (^Acacia), one of Lycium and 

 of Celtis, and the creosote bush iCovillea). The first two named are the 

 more prominent elements. The ocotilla has the form of an inverted cone, 

 the spreading, tapering branches forming the sides, and the single short 

 base the apex of the cone. When there is sufficient water in the soil the 

 branches are thickly studded with rosettes of slightly glaucous, rich 

 green leaves. The flowers, scarlet in color, are borne in a rather large 

 inflorescence at the ends of the branches, and when in flower in early 

 spring the plant is an unusually fine sight. The bark of the ocotilla 

 is very waxy, burning like a candle, and is therefore useful in starting 

 a fire. 



The paloverde is a small tree, branching from the base with irregular 

 spreading branches. Its bark is green, by which fact the Spanish name 

 is suggested. The smaller branches gradually taper into spines and 

 bear few small compound leaves. These are more frequently absent 

 altogether, but the plant is nevertheless able to carry on its work. 



All these plants, and many more which space daes not permit of men- 

 tion, are awaiting the investigator, in a rare climate, with a wide sweep 

 of country, of mesa and mountain, stretching as far as the eye can reach, 

 with every necessity at hand — a unique opportunity for the inquiring 

 botanist. 



ORIGIN OF PARASITISM IN FUNGI. 



Some very interesting experiments have been made in England by 

 Massee, by which he has been able to cause certain fungi to change their 

 behavior in a remarkable way. A fungus which under normal conditions 

 is parasitic on one species only may be induced to penetrate the tissues 

 of the leaf of a strange host by injecting the leaf with the juice extracted 

 from the usual host or with a particular substance, which in some cases 

 may be separated from the extracted juice. Similarly, fungi which usually 

 feed upon dead plant materials may be educated to become parasitic. The 

 explanation of this phenomenon is found in the fact that the germ-tubes 

 of a particular fungus are positively sensitive toward a certain substance ; 

 and if this is present in the leaf they will penetrate the tissues unless there 

 is some other substance present toward which they behave negatively. 

 For example, Botrytis chierea, which is a very common parasite on many 

 plants, can not attack apples because of the presence of malic acid, 

 although saccharose, toward which the fungus is positively sensitive, is 

 present. 



It is held by the author that immunity to the attack of fungi enjoyed 



