xii] NON-RADIAL ANATOMY 169 



attracted the attention of botanists^ the occurrence, namely, 

 of little colourless cellular plates, arising generally at the apex 

 and base of each lobe, but sometimes elsewhere (Fig. no y^and 

 5, p. 170). The cells at their base (c in Fig. no B) become 

 corky at an early stage, and the plates drop off. They are prob- 

 ably best interpreted as caducous trichomes; their function, if 

 they have one, is quite unknown. 



As examples of the flat, non-radial type of submerged leaf, 

 Callitriche^ Elodea and Alisma may be mentioned. In Fig. 1 1 1, 

 p. 1 70, the contrast between the aquatic and aerial leaf of Cal- 

 litriche verna is indicated. The water leaf is thin, but still retains 

 some mesophyll; the outlines of the epidermal cells in the two 

 forms show the distinguishing characters to which reference 

 has already been made. Callitriche autumnalis'^^ which lives and 

 flowers completely submerged, has a thinner leaf. The leaf of 

 Hottonia resembles that of Callitriche. The ribbon-leaf of 

 Alisma Plantago shows a slightly different type of structure. 

 The chlorophyll-containing epidermis forms the essential part 

 of the leaf, and the large air passages are bounded by it. There 

 is one main bundle, accompanied by two tiny laterals placed 

 close to the margins. In Elodea canadensis (Fig. 106, p. 165) 

 we reach almost the ultimate phase in reduction of the meso- 

 phyll, for here the entire assimilating tissue is reduced to the 

 two epidermal layers. The extremely delicate leaf is strength- 

 ened by some fibrous cells. Supporting sclerenchyma is cha- 

 racteristic of a certain number of submerged leaves such as 

 those of the Potamogetons (e.g. Fig. 38, p. 61). 



There is a strong tendency, in submerged leaves, to the 

 reduction of the tracheal system. Among the Hydrocharitaceae, 

 for instance, though typical spiral tracheids occur in the sub- 

 merged leaves of Stratiotes^ the leaves of a number of other 

 genera show either no tracheids at all, or else more or less 

 ephemeral elements with annular thickenings, e.g. Elodea^ 

 Halophila^ Vallisneria and Thalassia^. 



1 Irmisch, T. (1859^), Borodin, J. (1870), Magnus, P. (1871), and 

 Perrot, E. (1900). 



2 Hegelmaier, F. (1864}. ^ Solereder, H. (19 13). 



