LEAVES 



553 



Y#^ 



Alpine plants are said to have smaller lacunae than do the otherwise 

 similar arctic plants. In some cases, as in he leaf of Allium and 

 in the stem of Equisetum, capacious air chambers develop without 

 much reference to external conditions (fig. 1028). 



The influence of external factors upon the development of air 

 spaces. — Tlie plasticity of lacunar tissues. — Lacunar tissues are ex- 

 tremely plastic, and commonly have re- 

 ciprocal relations with palisade tissues. 

 For example, in the water leaf of 

 Proserpinaca, the entire mesophyll is 

 composed of lacunar tissue (fig. 769), 

 while the air leaf is composed of pali- 

 sades above and of lacunar tissue below 

 (fig. 768). In Lactuca scariola there are 

 possible all variations from a mesophyll 

 composed entirely of sponge to a meso- 

 phyll composed essentially of palisades 

 (figs. 770, 771). When the phellogen 

 or cork cambium (p. 705) of certain 

 swamp plants (as Jussiaea and Deco- 

 don) is submerged, it develops into a 

 loose lacunar tissue, known as aeren- 

 chym'a (fig. 793), whereas in the air it 

 develops into cork. In each case the 

 mature tissue is made up of cells ar- 

 ranged in radial rows, but in aerenchyma 

 capacious air spaces are interspersed 

 regularly, while cork is remarkably free 

 therefrom. Furthermore, in the aeren- 

 chyma the cell walls are thin and of 

 cellulose. The great development of 

 air spaces gives the stem a swollen appearance, and frequently the 

 aerenchyma grows so rapidly as to break through the bark, forming 

 whitish patches; the so-called water lenticels are similar scattered patches 

 of whitish tissue, formed under identical conditions (p. 663). 



Fig. 793. — A cross section of the 

 submersed part of a stem of Jussiaea 

 peruviana, showing the development 

 of aerenchyma (a) from phellogen {p) ; 

 note the capacious air spaces ; highly 

 magnified. — From SCHENCK. 



The factors involved. — The exact factors causing the development of lacunar 

 tissue are not known, though it is evident that the dominating influence is external. 

 The essential feature to be explained is the development of lacunae, since the cell 

 shape may remain much as in embryonic tissue, contrasting with the great change 



