24 ROOT HABITS AND PARASITISM OF KRAMERIA CANESCENS. 



Some theoretical interest is attached to the origin of the haustoria, or 

 more especially to their morphological nature. Goebel has made the point 

 that haustoria are probably organs sui generis; that is, that they do not 

 arise from previously existing' organs, as by the transformation of root- 

 primordia, but as new formations. (Organography of Plants, pt. 2, English 

 edition, p. 224.) He cites numerous instances in support of his view. 

 So far as the results at hand represent the actual condition obtained in this 

 parasite, it would appear that his conclusion may be too sweeping. As 

 shown in the preceding paragraph, haustoria in Krameria seedling's are 

 formed by the transformation of root-tips and hence are not new formations. 

 In older plants, however, the haustoria may be new organs, but this is not 

 proved . 



In Santalum and in Melampyrum haustoria appear on young roots of 

 seedlings and their formation is not dependent upon the presence of the 

 host- vSo far as my observations show, Krameria does not organize haustoria 

 unless the roots of a possible host mingle with its own. The period of inde- 

 pendent existence of the parasite, therefore, is apparently indeterminate. 



The roots of most of the desert perennials which are not provided with 

 special water- storage organs penetrate the ground as deeply as the mois- 

 ture conditions of the soil, or its structure, permit. Plants of this character 

 have generalized root-systems; plants with fleshy shoots, or fleshy roots, 

 are usually provided with specialized types of root-systems. These do 

 not change with change of substratum, but the generalized type is quite 

 variable. That is, if such plants with generalized root-systems occur on 

 bajadas where the soil may be less than half a meter in depth, the roots 

 accommodate themselves to the shallow soil; but if the same species grow 

 along the river, or the wash, where the soil is of indefinite depth, the 

 roots may attain a depth of 2 meters, or frequently much more. An ex- 

 ception to these conditions is to be found in Krameria, which does not have 

 water-storage facilities and still has a specialized type of root-system; 

 that is, the roots are superficial, whatever maybe the character of the soil, 

 This feature occurs in every habitat in this vicinity, irrespective of soil or 

 exposure conditions, and with its specialized distributional character is in 

 sharp contrast with those of the autophytes inhabiting the locality. 



