266 de Fraine. — The Morphology and Anatomy of the 
A comparison of A, B, C, D and E in Text-fig. 14 shows that the 
amount of sclerenchyma and the number of sclereide groups diminish 
considerably in the mud and culture plants as compared with the main bank 
form and the narrow-leaved lateral one, while the broad-leaved plant shows 
less marked differences, though the stereome is distinctly reduced. 
In the xylem of the sand and mud plants there is a little less 
fibrous tissue, and slightly more wood parenchyma is developed. 
In the culture plant no fibres occurred in the xylem of a two-year 
old plant, and in a narrow-leaved lateral of a similar age the wood was also 
free from fibres ; but in the lateral plant the wood was much more com- 
pact than in the culture form, in which considerable wood parenchyma was 
present. 
Culture, as in the case of the root, has increased the relative width 
of the phloem zone. 
Finally, differences occur in the size of the vessels (see Table II), but 
since only two-year-old culture plants were obtainable the measurement of 
the secondary vessels of this stem should not be compared with those 
of other plants, for increasing age probably means increasing diameter 
of the vessels up to a certain extent. 
These variations in the different forms are in entire agreement with 
those found in the root, and since the short stem is completely subterranean 
they are explicable on the same grounds. 
1 ' 
4. The Leaf. 
The general distribution of the tissues in the leaf of the narrow-leaved 
binervosa is shown in Text-fig. 15, A. Numerous small vascular bundles 
are present, and the mid-vein is encircled by a sclerenchyma sheath (se.) ; 
a large number of sclereide groups varying in size occur, and form a charac- 
teristic feature of the leaf. Comparison of the proportion of stereome 
present in the leaf blade of a narrow-leaved plant from the lateral banks and 
plants from the other habitats shows that in the sand form the sclerenchyma 
sheath round the mid-vein is much less developed, and the number of 
sclereide groups distinctly fewer (Text-fig. 15, B) ; the mud form shows 
similar features (Text-fig. 15, c) ; and they are still further emphasized 
in the plants from the experimental area (Text-fig. 15, D). The broad- 
leaved form has the sclerenchyma sheath confined to a cap above and below 
the main veins, and the sclereide groups are very few in number (Text- 
fig. 15, e), features which are interesting in comparison with the leaf of 
N. bellidifolia (Text-fig. 15, F), in which also there are only sclerenchyma 
caps to the main vein, and no sclereide groups at all. Comparison of 
Text-fig. 15, A, E and F, clearly demonstrates the intermediate character 
of the putative hybrid and the two suggested parents. 
