1 88 AERATING SYSTEM [ch. 



aerenchyma of the lenticels^. Salix viminalis and Eupatorium 

 cannahinum^ ag^i^j have been shown to develop spongy tissue 

 beneath the lenticels when grown in water or on marshy soil'^. 

 In the course of evolution, this tendency to hypertrophy of the 

 lenticel tissue under the influence of water, may have formed 

 the starting point for the development of the special air-con- 

 taining phelloderm which is so marked a feature of a number 

 of plants to which we must now refer. 



It was recorded more than forty years ago, by a Russian 

 observer, that the stems and roots oiEpilobium hirsutum^ Lycopus 

 europaeus^ and two species of Lythrum produced aerenchyma, 

 when grown in water^. In Lythrum Salicaria the aerenchyma, 

 which appears on the submerged parts when grown in shallow 

 water, enlarges the stem to as much as four times its normal 

 thickness^. It can be induced to occur in this and other cases 

 (e.g. Lycopus europaeus) by merely keeping the cut branches 

 in water for a few weeks i. The list of our native waterside plants, 

 in which aerenchyma occurs under suitable conditions, includes 

 Lysimachia^ Lotus^ Oenanthe^ and Scutellaria^ in addition to the 

 genera already named*. Schenck^, to whom our knowledge of 

 aerenchyma is largely due, showed that this tissue was particu- 

 larly characteristic of Onagraceae, where it occurred in twelve 

 species belonging to three genera; Leguminosae, where it was 

 found in six species representing five genera; and Lythraceae, 

 where it appeared in six species belonging to three genera. It 

 was Schenck who proposed the useful term 'aerenchyma' for 

 this non-suberised ventilating tissue produced by a phellogen. 

 The cells are not dead and empty, as in normal cork, but are 

 lined with a delicate protoplasmic pellicle and generally contain 

 clear cell-sap; they are separated by extensive lacunae. That 

 they are homologous with cork-cells is indicated by the fact 

 that, in the roots of Jussiaea^ the cork, formed when the plant 

 grows on land, is replaced by aerenchyma when it grows in 



1 Goebel, K. (i 891-1893). 2 Schenck, H. (1889). 



^ Lewakoffski, N. (1873^); on Epilohium see also Batten, L, (19 18). 

 ^Gluck, H. (1911). 



