18 ROOT HABITS AND PARASITISM OF KRAMERIA CANESCENS. 



So far as has been observed, the root-system of Parkinsonia microphylla 

 is of the generalized type, although it never attains "typical" develop- 

 ment on account of the character of the soil where it grows, which is not 

 more than 50 cm. in thickness and is underlaid by rock or by impervious 

 caliche. A stout tap-root reaches to the caliche and a few large laterals 

 run out through the upper soil-layers for an undetermined distance. The 

 latter branch infrequently. The parasitic relation occurs on small sec- 

 ondary or tertiary roots. 



Haustoria were seen on roots of Parkinsonia from 1 to 3 mm. in diameter. 

 The detail of the relationship is as follows: A root 1 mm. in diameter bore 

 2 haustoria 1.6 mm. apart; another root 3 mm. in diameter had 6 attach- 

 ments of the parasite within 10 cm.; one 2.5 mm. in diameter bore 3 haus- 

 toria 1.1 cm. apart. 



Sections of the host root at a point where the haustorium was attacked 

 revealed a condition not seen in any other host. The haustorium pen- 

 etrated through the cortex and reached the central cylinder, and, except 

 for a short distance in the cortex, was covered with cork (fig. 5, plate 4). 

 That is, the parasite was ciit off from the woody cylinder of the host, from 

 which, as a consequence, it did not derive benefit. 



Cross-sections of the root of the host, made through the haustorium 

 also, show the ducts adjacent to the haustorium to have been plugged. 

 This appearance was not seen in any other host, and its reason is not 

 known, but it may be associated in some manner with the fact of parasit- 

 ism, although plugged ducts were seen also at some distance away from 

 the point of haustorial attachment. (Fig. 5, plate 4.) 



Prosopis Velutina. 



The mesquite, Prosopis velutina, in the vicinity of Tucson, occurs mainly, 

 and abundantly, on the flood-plain, where it attains a height of 10 to 20 cm. 

 and forms a forest. It is found also, to a limited extent, on the lower 

 slopes of the low mountains, and in hollows, depressions, and drainage- 

 basins on the bajadas where the soil is of sufficient depth. It is not to be 

 found where the hardpan caliche comes to within 10 to 20 cm. of the sur- 

 face, as it usually does on the slopes. The association of Krameria with 

 Prosopis was seen in the alluvial flat west of Tumamoc Hill. 



The position of the root-system of mesquite in the ground varies con- 

 siderably. Where the soil is deep, as on the flood-plain, the roots, as a 

 rule, go deeply, but run only a short distance horizontally. Where the 

 soil is shallow, or where there is surface irrigation mainly or exclusively, 

 the laterals may extend as far as 25 meters very close to the surface of the 

 ground. A striking feature of roots of many desert plants, not exemplified 

 by the mesquite, is that they either go directly down or take a horizontal 

 position, and not, as a rule, an angle between. The parasitic relation was 

 observed in connection with one of the more shallowly placed laterals. 



