xx] AERATING SYSTEM 259 



It has been suggested by Goebel 1 that the origin of the 

 development of intercellular spaces in water plants may be 

 attributed to the direct action of the medium an enlargement 

 of the air spaces resulting mechanically from the pressure of the 

 gases evolved, which are prevented by the surrounding water 

 from escaping freely. But he points out that the lacunar system, 

 thus initiated, has ultimately become hereditary. Some support 

 is given to Goebel's view by experimental work on amphibious 

 plants, and by the study of the comparative anatomy of speci- 

 mens growing under different conditions. It is found, for 

 instance, that if such a plant as the Water Speedwell, Veronica 

 Anagallis, grows with one of its shoots submerged, while the 

 others develop in the air, the submerged shoot shows an in- 

 crease in intercellular spaces, as compared with the air shoots 2 . 

 But the presence of lacunae is something more than a mere 

 direct effect of environment, since they persist, even if in a di- 

 minished form, when aquatics are grown on land. For example, 

 stems of Pe-plis Portuta, when grown in water, are characterised 

 by four large lacunae in the cortex. On examination of plants 

 growing terrestrially, it has been found that they also show four 

 lacunae; the only difference between the aquatic and aerial 

 plant is that, in the former, the bands of tissue separating the 

 main lacunae are riddled by intercellular spaces, while, in the 

 latter, they are relatively solid 2 . 



Whatever its origin may be, the aerating system in the stems, 

 leaves and roots of water plants belonging to the most divergent 

 cycles of affinity, is developed with a uniformity and an elabora- 

 tion which undoubtedly indicate that it is definitely related to 

 the milieu*. It is perhaps scarcely too much to say that the 

 difficulty of breathing is the principal drawback to life in water, 

 and that only those plants which have an inherent capacity for 

 coping with this difficulty, can make their home permanently 

 in an aquatic environment. 



1 Goebel, K. (1891-1893). 2 Costantin, J. (1884). 



3 For a consideration of the aerating system from the anatomical 

 standpoint see Chapter xiv, p. 183. 



172 



