72 



THE FACTORS 



[Part I 



Fig. 45. Caperonia hetero- 

 petaloides, Miiller Arg. Trans- 

 verse section through the stem, 

 with a sheath of aerenchyma. 

 Natural size. After H. Schenck. 



free by the assimilating cells to the points where it is consumed in the 

 non-green parts that are respiring 1 . 



Woody plants whose roots and stem-bases are in stagnant and therefore 

 badly aerated water, are provided with special means for obtaining oxygen 

 from the atmosphere. Thus the base of the stem of many swamp trees is 



greatly swollen and, owing to the disruption of 

 the tissues, is hollow in its centre ; the cavity 

 serves as an air-reservoir and communicates with 

 the atmosphere by means of intercellular spaces 

 and lenticels. As a rule, however, certain special 

 tissues, or even entire members of the tree, serve 

 to supply oxygen and exhibit an organization 

 suitable for the purpose. 



Aerenchyma' 1 ^ which was first accurately de- 

 scribed and had its functional significance clearly 

 explained by H. Schenck, is extremely common in 

 woody plants growing in wet soils : this tissue is homologous with cork, but 

 completely differs from it both in histological and oecological characters. 

 In many plants it surrounds those woody portions of the stem and roots 

 that stick into wet soil, with a thick, spongy, fissured mantle (Fig. 45), 



which occupies the position of the ever- 

 absent cork and is bounded by phellogen. 

 This aerenchyma (Fig. 46) consists of thin- 

 walled, non-suberized cells, loosely united, 

 and bounding broad intercellular passages 

 which form a continuous and much ramified 

 aerating system. The air-passages ter- 

 minate in the numerous external fissures 

 and open directly to the water, which 

 does not enter them. The aerenchyma is 

 not confined to the wet parts, but extends 

 over parts above the surface of the water. 

 Yet traced upwards into the air it rapidly 

 decreases in thickness and passes over into 

 ordinary cork. Sometimes the formation 

 of aerenchyma is confined to the lenticels, 

 out of which it projects in a cauliflower-like form, whilst the remaining 

 phellogen produces typical cork even under water. 



The aerating tissues of woody plants do not always originate in the 

 phellogen. In many cases the greatly developed primary cortex, traversed 

 by broad air-canals, serves for the transport of oxygen, which is partly 



Fig. 46. Caperonia heteropetaloides, 

 Miiller Arg. Aerenchyma of the stem 

 in transverse section. Magnified 96. 

 After H. Schenck. 



1 See H. Schenck, I ; Goebel, II, Bd. 2, Aquatic Plants. 



2 Schenck, II. 



