MacDougal: tubers of isopyrum biternatum. 503 



layer, and are most delicately sensitive to the metabolic con- 

 ditions prevalent in the cell, to which they respond by changes 

 in size, form and structure. These cells serve for the storage 

 of reserve food — principally carbohydrates as will be des- 

 cribed below. The tension of the expanding tissue derived 

 from the pericycle induces a tangential expansion of both en- 

 dodermis and cortex. In the endodermis this has been accom- 

 plished by radial longitudinal division, and in many of the cells 

 three or four secondary walls have been formed. The cortex 

 which in normal roots may attain a thickness of 8 to 12 layers, 

 in the tubers is rarely more than 4 to 6 in thickness, due to the 

 expansion in a tangential direction. In both cortex and en- 

 dodermis the secondary can be distinguished from the primary 

 walls by their non-suberization. The division of the cortex is 

 not so regular as in the endodermis, and portions of the outer 

 layers are lost by decay. In some instances patches of the 

 piliferous layer remain. The endodermis and outer layers of 

 cortex contain large pale gray and yellowish brown globules 

 and masses whose composition will form the subject of a sep- 

 arate paragraph. The rays extending outwardly from the 

 secondary tissues, reach one half to two-thirds of the distance 

 to the endodermis, and are composed of cambiform cells which 

 are clearly meristematic except in some instances at the outer 

 edge where in a small group the protoplasmic content has been 

 partially lost, the walls thickened and pitted and a trace of 

 lignification has appeared. The rays comprise two or four layers 

 of cells, which in the more external portions exhibit a greater 

 radial than tangential diameter. The parenchymatous tissue 

 lying in the plane of the rays exhibits a radial arrangement 

 similar to that of the rays. In the thickenings of the roots of 

 Isopyrum trifoliatum which are triarchic, similar wedge-shaped 

 extensions of cambium tissue occur, and one or more vessels 

 may be formed at the outer edge of the ray. When the forma- 

 tion of a tuber occurs in a portion of a root from which a branch 

 arises, the thickening entails a disposition of the tissues which 

 is most clearly seen by reference to Fig. 11 PL 29. The thick- 

 ened woody nature of the cells at the outer edge of the ray is 

 preserved in the lateral converging branches. 



In I. biternatum the small mechanical value of the woody 

 elements is supplemented by the high degree of turgidity of 

 the comparatively large mass of storage tissue; a turgidity 

 resulting in part from the high osmotic coefficient of the con- 

 tained sugars, but maintained even when free from reserve 



