218 The Phenomena of Morphogenesis 



( 1908 ) sliced off a portion of a kohlrabi tuber, the living cells at the new 

 surface differentiated into a rather typical epidermis in which even 

 stomata were formed. In roots of the Araceae and air roots of orchids, 

 where there is no cell division after an injury, parenchyma cells near a 

 newly exposed surface redifferentiate into thick-walled ones essentially 

 like those of a normal hypodermis. 



Even more complex patterns may be reconstituted under the influence 

 of a different environment. In the roots of Philodendron Glaziovii there is 

 a row of brachysclereids a few cell layers below the surface. After the 

 experimental removal of the outer tissues, a similar row of thick-walled 

 cells differentiates at about the same distance below the new surface 

 ( Bloch, 1926 ) . In the air root of Monstera, the cells of the cortex normally 

 remain undifferentiated for a considerable distance back from the tip. 



Fig. 8-22. Air root of Philodendron. Below arrow, 

 normal hypodermal tissue pattern, with layer of 

 brachysclereids. Above, regeneration of similar 

 layer below wound. (From Bloch.) 



At this point, however, the four or five cell rows next the outside often 

 form thick, lignified walls and develop into brachysclereids. The differ- 

 ence between these cells and their unlignified neighbors is not evident at 

 the last cell division nor can it be traced through any cell lineage. It arises 

 as these two types of cells become mature. The occurrence of lignification 

 is apparently related to the position of cells with reference to the surface 

 of the root and thus probably to such an environmental factor as an 

 oxygen or water gradient. When a root of Monstera is wounded in such a 

 manner that the parenchyma cells of the inner cortex are now exposed to 

 a new, artificially produced surface, they become thick-walled brachy- 

 sclereids (Bloch, 1944; Fig. 8-22). When Wardlaw isolated the central 

 core of the shoot meristem by vertical incisions, he observed that the 

 cylinder of vascular tissue regenerating inside the core developed at a 

 constant distance from the new surface made by the cuts ( p. 238 ) . 

 In a few instances where the normal ontogeny may be completed under 



