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JOUENAL OF THE EOYAL HORTICULTURAL SOCIETY. 
epidermal layers and the veins. Floating leaves, it need hardly be 
remarked, will still more effectually exclude light from those that are 
submerged ; and this tends to bring about a physiological division of 
labour in those cases of heterophylly, or the occurrence of two kinds of 
leaf on one plant, to which I shall presently again allude. It is no doubt 
the demand for light that brings about the ribbon -like elongation of some 
submerged leaves. Thus in Sagittaria, whilst those leaves, usually the 
majority, that project above water are arrow-shaped, floating leaves have 
an ovate blade, and fully submerged ones are ribbon-like. In swift 
currents the leaf-blade is sometimes suppresed, the petiole forming a flat, 
limp pale strip 70 centimetres (28 inches) long, and 1-2 centimetres 
broad, which might easily be mistaken for the leaf of Vallisneria. * 
In Callitriche, the Water Starwort, the submerged leaves are longer 
and narrower the deeper they are below the surface ; and in Hippuris, 
the Mare's-tail, whilst the aerial leaves are only from seven to nine times 
as long as they are wide, the flaccid submerged ones are thirty times their 
width, t 
One of the most important aspects of water as a medium for plant life 
is the presence in it of dissolved gases. Though many so-called aquatic 
plants are not free-swimming or floating organisms, but are rooted to the 
bottom and may obtain part of their food in this way from the soil, the 
greater part of their food is undoubtedly in solution. Decomposing 
animal and vegetable bodies give rise to ammonia, a highly soluble source 
of nitrogen for submerged plants ; whilst, owing to the difference in the 
solubility of oxygen and carbon-dioxide, water contains less oxygen and 
more of carbon dioxide in proportion than does the air. Since air con- 
tains more oxygen and less carbon dioxide than is desirable for the most 
favoured vegetation, the water-plant has in this respect an advantage. 
Correlated with this presence of gaseous food throughout the medium 
surrounding them is the absence of cuticle, the copious branching of stem 
and leaf, and the reduction or absence of roots in aqueous plants. The 
numerous and large intercellular spaces also, which are so marked an 
anatomical feature in aquatics, though undoubtedly they may often 
perform the distinct function of acting as floats, buoying up the plant 
to the light at the surface of the water, are connected with gaseous 
nutrition. 
Water plants [says Sachs %] are of peculiar interest, because their intercellular 
spaces do not open outwardly through numerous stomata, but communicate with large 
cavities which are formed in the interior of the tissues by the disjunction of cells or 
[less commonly] by their union with one another by the rupture of their walls. The 
underground stems of Equisetum and of many bog-plants show similar phenomena. 
Uninjured plants of this kind are closed and air-tight outwardly : the gases which 
collect in the cavities can originate only from the surrounding tissues, which absorb 
oxygen, nitrogen, and carbon-dioxide by diffusion from the surrounding water. These 
gases cannot at once diffuse through the surrounding tissues, but undergo change 
within them, and when once collected in the spaces they are still further influenced by 
the chemical changes that go on in the surrounding tissues. A submerged water- 
plant, for example, which contains chlorophyll absorbs carbon-dioxide from without 
under the influence of sunlight ; and at least a portion of the disengaged oxygen 
collects in the cavities. When it becomes dark this process ceases. The collected 
* Goebel, Pfianzenhiologische Schildenmgen, ii. 290, and Kerner, 02^. cit. ii. 502. 
f Kerner, op. cit. ii. 505. 
j Julius Sachs, Lehrbttch der Botanik, first English edition (1875), pp. 615, 616. 
