268 de Frame.' — The Morphology and Anatomy of the 
but it also has a marked effect on the thickness of the walls of the ele- 
ments, as can be seen on comparison of A with C and B with D in Text- 
fig. 17. This again is in entire agreement with the results obtained by 
Starr in her comparison of mesophytic and dune forms (see p. 262 ). 
In the leaf sheath of all the forms the stereome increases very con- 
siderably in amount ; the walls of the elements become much thicker, and 
their length increases greatly ; the sclereides become long, fibre-like elements 
(cf. Text-fig. 17, B and I) = typical petiole sclereides, with Text-fig. 18, 
A and B = typical sheath sclereides). 
Text-fig. 16. Diagram of the transverse section ot the middle region of the petiole of S . 
binervosa (a-c) and S. bellidifolia (d). a = narrow-leaved plant ; l) = broad-leaved plant; c = narrow- 
leaved plant cultivated from seed. x 30. Stereome in black ; vascular bundles dotted : sc. 1 . — 
sclereides; sc. = sclerenchyma sheath. 
The detailed structure of a narrow-leaved form growing on a lateral is 
shown in Text-fig. 19, A: on the upper surface is a well-marked palisade 
layer two or three cells deep ; these pass over gradually into the rather 
shorter palisade-like cells of the lower surface ; practically no intercellular 
spaces are developed. Chlorophyll occurs in all the cells except those 
of the epidermis : where red coloration is present it is due to red cell- 
sap. The leaf of a Main bank plant is precisely similar in all respects 
to that of a narrow-leaved lateral one. 
The leaf of the mud plants is on an average slightly thicker than 
that of the typical form, the increased thickness being due to elongation 
