536 



PHYSIOLOGY. 



diately upon the "vascular ring and breaking through the cortical 

 parenchyma. 



The "woody adventitious roots of arborescent Monocotyledons differ 

 only in the greater development of the fibre-vascular structures; and 

 they emerge from the stem (Palms) in the form of thick conical shoots. 



When adventitious roots, like those just described, die away, they 

 decay down to their very origin, and leave a scar in the form of an orifice 

 surrounded by the ragged coleorhiza. 



In the thickened adventitious roots of Asparagus, which perform the 

 function of tubers, the parenchyma is greatly developed. In the tuberous 

 roots of Orchids (figs. 21 & 22) the central woody axis becomes irregularly 

 expanded into parenchymatous tissue driving 'the vessels out nearly to 

 the periphery, so that the characteristic structure is greatly disguised. 

 The aerial roots of the epiphytic Orchids have the growing extremities 

 clothed by several layers or a parenchymatous tissue, in which the cells 

 are characterized by delicate open spiral-fibrous secondary layers. 



Roots of Dicotyledons. In these plants the root has at first the same 

 structure and arrangement of its elements as in Cryptogams and Mono- 

 cotyledons, the rootlets being formed opposite the vessels from the peri- 

 cambial layer. A great difference, however, shows itself in a secondary 



Fig. 582. 



Fig. 583. 



Fig. 582. Extremity of the root of a germinating Turnip, with root-hairs. Magn. 30 diam. 

 Fig. 583. Longitudinal section through young root, showing the root-cap. 



formation of liber and vessels, which enables the roots to thicken and 

 even to form concentric zones exactly as in the stem. In the early stage 

 of the Dicotyledonous roots the bundles of liber-cells and the bundles of 

 vessels are, as in the other groups of plants, alternate with each other, 

 but a secondary formation of cambium-cells takes place on the inner side 



