RHIZOMES AMD HOOTS. 211 



2. Monocotyledons. 



From this class numerous official rhizomes are derived. The 

 epidermis is usually succeeded by a rather thick layer of cork, 

 within which there is a parenchymatous cortex traversed 

 ongitudinally by fibrovascular bundles (leaf traces), and 

 separated from the stele by the endodermis. The stele is com- 

 posed of parenchymatous tissue, throughout which numerous 

 fibrovascular bundles are distributed. These bundles are usually 

 more closely approximated to one another near the endodermis 

 forming a more or less dense ring, while towards the centre of 

 he stele they are scattered. Although one bundle may fuse 

 with another, they do not all unite to form a single ring of wood 

 bundles surrounded by a ring of bast as they do in dicotyledo- 

 nous rhizomes, nor is any merismatic layer (cambium) produced. 

 Monocotyledonous roots also have a distinctive structure. 

 Within the outermost layer or epiblema there is a cortex com- 

 posed of parenchymatous cells often filled with starch ; the 

 endodermis is usually distinct and often coloured ; the stele 

 contains a number of fibrovascular bundles so closely approxi- 

 mated to one another as to form within the pericycle a con- 

 tinuous ring which is not traversed by regular bands of parenchy- 

 matous cell? analogous to medullary rays. Within the ring of 

 fibrovascular bundles there is usually a more or less well de 

 veloped pith. 



Dicotyledons. 



Dicotyledonous rhizomes commonly exhibit a tegumentory 

 tissue composed of several layers of cork cells within which is 

 a parenchymatous cortex traversed longitudinally by 

 .fibrovascular bundles (leaf traces). The endodermis is some- 

 times distinct, but sometimes it is difficult to identify. The 

 : tele differs from the stele of monocotyledonous rhizomes in 

 the nature and distribution of the fibrovascular bundles it 

 contains ; in dicotyledonous rhizomes these are usually 

 arranged in a single circle and often closely approximated, 

 being separated by regular strands of parenchyma (medulla y 

 rays). Between each wood bundle and the pericycle there is a 

 group of bast tissue, and between the bast and the wood a layer of 

 merismatic tissue which ultimately forms a complete 

 cambium ring. 



During the earlier stages of their growth, dicotyledon c us 

 hizoms may easily be distinguished from roots, as the formcc 

 possess the primary structure characteristic of ste-ns and th 

 latter that character istic of root-. As the age of these orgcin . 

 . increases, it becomes increasingly difficult to distinguish wi h 

 certainty between them, for the primary structure becomes 

 more and more obscure until it is finally undiscerni >le. Th's 



p 2 



