ROOT-SYSTEMS OF PERENNIALS. 



51 



with the position wliich it occupies relative to other plants. When it is 

 isolated, the shoot is rounded and much-branched; where the plant is pro- 

 tected by another plant, the branches are few and may be much elongated, 

 and we shall see that the roots also of the plant vary with its position. 



I'iG. 5. — Root-systems of Opuntia h'l>lncaxilis. a, plant growing at the base of Acacia con- 

 stricta. l\ plant growing apart from other species. Both figures are of mature plants. 



Another character of the species is the presence of short branches, shown 

 in plate 10, which constitute the chief chlorophyll-bearing organs. Under 

 favorable water conditions, these short branches may be very numerous, 

 but where the moisture conditions are otherwise, they are few. The 

 branches really function as leaves, in that they are formed at times of active 

 growth and that their subsequent history varies with the moisture relations ; 

 if favorable, they are long retained; if unfavorable, they early fall away. 

 This effect can be quickly achieved by uprooting a plant and hanging it in a 

 dry place, under which treatment the short branches to a large extent soon 

 drop off. The exfoliated branches do not commonly, if ever, act as propa- 

 gative bodies, as is the case in otherspecies, notably in O. vhi para .2iSvi\\\ be 

 shown below. 



The root-systems of two specimens of Icplocauliswx-K studied inl'ebruary, 

 1907; one was growing at the foot of Acacia constricta (fig. 5a) and the 

 other independently (fig. 56). Both plants were on the northern slope of 

 Tumamoc Hill tmder the same conditions of soil and moisture as described 

 above for Encclia. The protected plant was 80 cm. high, and the sub-aerial 

 portion consisted of a central axis with a few long slender branches that 

 carried many short ultimate ones of the character spoken of in a preceding 



