KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55. N:0 5. 



123 



does the epidermis get a supporting stratum as in the Coleogeton-species nor have 

 stomata been perceived. At the same time as that transformation takes place the 

 leaf-apex rounds itself so that the best developed of them have a fully rounded 

 apex (Fig. 55 B, C, D). Indeed, here we have the very first beginning of an evo- 

 lution of floating leaves. The next step is the evolution of palisade-tissue and sto- 

 mata and a greater difference between blade and stalk. This takes place in the 

 Javanici, having in this respect, 

 therefore, reached a higher stage 

 than has P. pusillus, and species 

 of the pusillus-group in general. 



The development of floating 

 leaves has its natural reason in 

 the necessity of holding the shoots 

 erect and of reaching the sur- 

 face of the water, which end the 

 plant in general and originally 

 attains by its lacunar system of 

 the stem and leaves. A still higher 

 degree of evolution is the trans- 

 formation of floating leaves into 

 air-leaves. 



The ligules of P. pusillus 

 are 6 — 8-nerved in the front-field 

 and always open and convolute 

 in contradistinction to P. panor- 

 mitanus and the other Pusilli 

 connati. 



The turios of this and re- 

 lated species are for the purpose 

 of classification of great import- 

 ance. In P. pusillus they con- 

 sist of transformed branch-tops. 

 The lower free leaves of the turio, 

 usually 3(2 — 4), are commonly 

 somewhat shorter and at their 



base a very little thicker than the other leaves of the branches, but in other 

 respects quite like. The upper part of the turio presents, by the amassment of 

 leaves and sheaths, a considerably incrassate and fusiform shape, and the sheaths 

 embrace quite the inner obtuse leaves. By this the species differs very clearly 

 from the otherwise nearly similar P. panormitanus. 



The pollen and pistils do not differ from the type of the group. The fruit, 

 in its fresh state with rounded back, gets often when dry a false keel. In size it 

 varies from 2 to 2,5 mm in length, in form as in the fig. 55, G is exhibited by 



Fig. 55. /'. pusillus L. A, a — h, Tops of typical stem-leaves showing 

 the different shape of the apex, '-f. R> C, Involncr.il leaves (floral leaves) 

 developed as primitive floating leaves, the lacnnar part dotted, ca. f. D, 

 Top of such a leaf, in form less differing from the original shape, \ E, 

 Transverse section of the leaf Ii at the middle, s , . F, Transverse section 

 of a common stem-leaf at the middle showing the nerves (»i, n'), strands 

 (str), and lacnnar system, (l), \°. G, Frnit, lateral view, showing the de- 

 marcation line of the operculnm. The dotted lines on the inner side and 

 at the base show the variations in form, ','. 



