246 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



become cell-surfaces in light of high intensity, whilst they continue 

 to grow as tubes if the illumination is feeble. Like phenomena are 

 found in a number of plants from the most different groups of the 

 Plant Kingdom. But we also observe the phenomena of flattening of 

 the illuminated side in organs where we can discover no utilitarian 

 explanation, as, for example, in the shoot-axis of plagiotropous mosses 

 (Fig. 113), in the climbing stems of some monocotyledonous plants; 

 in other cases however the use of the flattening and of the increase of 

 surface (which is not always a necessary consequence) is quite evident. 

 The roots of some monocotyledonous and dicotyledonous plants furnish 

 instructive illustrations. 



Normal radial roots without chlorophyll are buried in the soil. But 

 the roots of some, although not all, plants have the capacity of becoming 

 green when exposed to the light, and this happens regularly in the aerial 

 roots of many Orchideae. and in Podostemaceae which possess roots. 



The roots containing chlorophyll in 

 both these families have often a form 

 different from that of the ordinary root ; 

 they acquire a dorsiventral construction 

 which in the case of the orchids is 

 exhibited particularly in the structure of 

 the velamen, and in addition the roots 

 often become flattened and to such an 



FIG. 122. Oenone Icptophylla, one of the 



Podostemaceae. Transverse section of the dorsi- extent that tllCV are quite leaf-like . 

 ventral green root. The ventral side which is 



fastened to the substratum by many hairs some When this flattening takes plaCC ill TOOtS 

 of which are indicated, is flattened, but the dorsal 



side is also somewhat flattened. The vascular lying Ott a Substratum, that Upon tile 

 bundle system is excentnc. J 



side of the root touching the substratum 



must be distinguished from that on the side directed to the light. The 

 former is brought about by the firm fixing of the root on its substratum, 

 such as stems of trees, stones 2 ; the latter, so far as there is a difference 

 between the illuminated and shaded side, is at least in many cases 

 caused by the light 3 . We see this in Sarcanthus rostratus and S. Parishii, 

 Epidendrum nocturnum, and Phalaenopsis amabilis. In the last-named 

 plant those of its roots which are buried in the substratum are cylindric 

 and radial, whilst those growing in light are flattened on the illuminated 

 side and the velamen there shows a structure affording a greater protection 

 against transpiration than is found upon the shaded side. In Angraecum 

 fasciola the dorsiventral aerial roots are not the result of the direct 

 influence of light, for they develop in this form even in darkness. 



1 The name Taeniophyllum tells that Blume mistook the flat green roots in this genus for leaves. 

 See my ' Prlanzenbiologische Schilderungen,' i. p. 194. 



2 See what is said about shoots on page 90. 



3 Janczewski, Organisation dorsiventrale dans les racines des Orchidees, in Ann. d. Sc. Nat., Ser. 7, 

 xii (1885) ; Goebel, Prlanzenbiologische Schilderungen, i. p. 197, and ii. p. 351. 



