• B 



ISOETES JAPONICA, A. BE. 



365 



tlie ligule, frequently abutting directly upon the ligular sheath, occur both in the fertile 

 and in the sterile leaves. They are not continuous with the tracheids of the leaf- 

 bundle, so that their function is probably not water-conduction but water-storage. 

 However, since these cells are also present in I. lacmfris, which is practically always 

 submerged, it is possible that their original function has been lost. 



Bud-Protection. 



We have already mentioned that the apex of the stem together with the youngest 

 leaves and leaf-rudiments are situated at the base of a comparatively deep dopressioQ in 

 the cortex (PL 33. figs. 7, 8). Thus they are surrounded on all sides by cortical tissue, 

 whilst above they are completely enclosed within a canopy formed by the overlapping 



fV^ 



Text-%, 20. 



Iso'etes jajjonka. Transverse section through leaf-rasette. The Hgules are darkly shaded- X 10 



7a 



mucilage. 



V-' 



of the older lea\ 



In some species, e. g. I. lacustriSy the wings of 



of sufficient width to completely enclose the next younger leaf (PI. 37. iigs. 52, 53, and 

 text-fig. 19). In this species the ligule is relatively very small. The wings of the leaves 

 of I. japo7iicay on the other hand, are not sufficiently broad, except at the very base of 



the leaf 



Their protective function, ho 



taken over by the ligule, which 





remarkably well developed in this species (text-fi 

 and thus protects, the younger leaves, but provide 



20) 



The ligule not only envelops 

 additional protective device ii 



the form of a copious secretion of m 



This sccietion of mucilage is particularly 



abundant in the youngest ligules, i. e. those found in the very centre of the leaf-rosette, 

 and therefore it usually happens that the actual apex of the stem is covered with a film 

 of mucilage (m in text-fig. 20). 



