AERIAL ROOTS 185 



piliferous layer and cortex are derived is represented at 

 the apex by a single initial layer of cells. 



4. Outside the dermatogen, at the apex of the root, 

 will be found another formative tissue, the cells of 

 which divide parallel to the surface of the dermatogen : 

 this is the calyptrogen-layer, which is formative of 

 the tissues of the root-cap. The latter appears as a 

 mass of parenchyma, covering the whole apex of the 

 root : the outer cells of it will be seen to be undergoing 

 disorganization, and mucilaginous degeneration of the 

 cell-walls. 



Aerial Roots. 



Examine fresh aerial roots of Vanda, Oncidium, or some other 

 Orchid, and note the greyish appearance of the outer band of 

 tissue or velamen, and the green colour of the tissues beneath : 

 also the smooth green apex of the root. 



Cut transverse sections from the mature part of the root of 

 Vanda : other epiphytic Orchids will do as alternatives, though 

 differing in minor details : mount in glycerine, and observe 



1. A peripheral band of tissue about five layers of cells thick, 

 which constitute the velamen : note the fibrous thickening of the 

 walls, and absence of contents in this covering of extra-fascicular 

 tracheides. 



2. At the inner limit of the velamen is a definite layer of 

 the exodermis, a protective tissue which is not uncommonly 

 found beneath the piliferous layer of roots which retain their 

 cortex. 



3. Within this a broad band of cortex, containing chlorophyll, 

 and many cells contain raphides. 



4. Centrally lies the vascular cylinder, having an inter- 

 ruptedly thickened endodermis, and alternating but ill-defined 

 groups of xylem and phloem : a few isolated groups of vessels and 

 sclerenchyma may be found scattered about the centre of the 

 cylinder, and this is not uncommon in large roots of Monocotyle- 



