250 Campbell . — Contributions to the 
very early stage. Very soon the tissue-systems become 
differentiated, and the root elongates rapidly and breaks 
through the overlying tissues. In its structure and growth 
it does not differ in any way from the first one. 
The stem has no fibro-vascular bundle apart from the 
common bundle formed by the coalescence of the bases of 
the bundles of the leaves and roots. In both leaves and 
roots there is but a single bundle. This in the leaves is very 
decidedly collateral in the arrangement of its tissues, with the 
xylem upon the inner (upper) side. Except for their larger 
size, and somewhat better developed fibro-vascular bundle, 
as well as having usually four rows of large air-spaces, instead 
of two, the full-grown leaves resemble in structure those first 
formed. 
The histology of the mature sporophyte was not investi- 
gated further, as this has already received full attention from 
other investigators. 
Owing to the large air-spaces in the leaves, they are much 
lighter than the water in which the plant grows, so that they 
finally stand upright, whether their first position was hori- 
zontal or vertical. In the former case, the apex then appears 
to be attached laterally, and the apex of the stem is horizontal. 
Longitudinal sections of young plants with two or three leaves 
(see Fig. 50) show that the stem has a flat, or slightly con- 
cave apex which lies at the bottom of a deep cavity formed 
by the bases of the leaves. The third leaf arises in the same 
way as the second, and at a point distinctly opposite, i. e. 
immediately above the first. 
The development of the young plant was not followed 
beyond this point, but probably agrees with Hofmeister’s 
account of /. lacustris \ in which the \ arrangement of the 
leaves continues up to about the eighth, when it is replaced 
successively by the J, §, f, x 5 g, and arrangements. 
1 The Higher Cryptogamia, pp. 354-356. 
