DIV. II 



PHYSIOLOGY 



283 



formation of a new one in the vicinity of the wound or the out- 

 growth of one which was in a rudimentary condition. Examples 

 of this type of restitution are afforded by the Algae and Fungi, and 

 especially by Bryophyta. These can only be mentioned here, and 

 consideration will be limited to the Flowering Plants. The capacity 

 to form roots is especially wide-spread. In Geraniums, Willows, 

 and many 'other plants, roots can be induced to form at any point 

 by cutting off the shoots. In other plants the roots develop at 

 particular places such as the older nodes. After roots have developed, 

 the stem gives rise to a complete plant either by the unfolding of 

 axillary buds or by the development of new growing points of shoots. 

 Separated leaves are often able to form 

 roots, though the power of giving rise 

 to a new shoot is rarely connected 

 with this. Even separated roots, when 

 they are able to give rise to shoots, 

 may regenerate new plants. Regenera- 

 tive buds may also arise on tendrils, 

 flowers, and fruits. When in regenera- 

 tion the production of shoots is not 

 provided for by existing growing 

 points, new ones may 

 be developed. If the 

 growing point of a 

 seedling is destroyed 

 a new growing point 

 may be developed from 

 the meristem above 

 the youngest leaf- 



\A hile FIG. 257. Transverse section of the leaf of Begonia showing the 

 the regeneration is development of an adventitious shoot from an epidermal cell, 

 a, The epidermal cell has divided once ; 6, a mnlticellular 

 nere restricted to men- meristem has been produced, (x 200. After HANSEN.) 



stematic cells, in other 



cases older fully-grown cells may recommence to grow and divide 

 and thus return to the meristematic condition. A special tissue, called 

 CALLUS, is thus first formed at the wounded surface, and new shoots 

 may form within this. In yet other cases fully-grown epidermal or 

 parenchymatous cells may give rise to growing points directly, i.e. 

 without the formation of callus. Fig. 257 shows the origin of a 

 regenerative shoot from an epidermal cell of a leaf of Begonia. 



Tissues may also be regenerated from mature parenchymatous cells. Thus 

 when the conducting tracts are interrupted new vessels may be formed from the 

 parenchyma and re-establish the connection. The tissues which have been removed 

 or interrupted are, however, not always formed anew ; frequently substitutionary 

 growth takes place. Thus, as a rule, an epidermis is replaced by cork, and it is 

 exceptional for a true epidermis with stomata to be regenerated ( 56 ). 



