'296 PLANTS OF NEW ZEALAND 



Where water is at all times available without stint, obviously, 

 an elaborate root system is not required. Hence, water plants 

 have few roots, and sometimes none at all. In the latter case 

 they float (e.g. Vtricularia protrusa) ; at other times the roots 

 are merely hold-fasts, for in many water plants there is a thin 

 epidermis, so that water can be taken in all over the whole 

 surface of the plant by osmosis. Consequently, also, water 

 conducting tissues in the stem are not required ; and, as no 

 firm tissues are needed to keep the plant upright, woody fibres 

 are always wanting in the stem, which is maintained in 

 position by the buoyancy of the water. In most water plants 

 there are large air chambers, which reduce the specific gravity 

 of the plant, and assist to float it. It is therefore very flaccid, 

 and, when removed from water, collapses. The leaves, too, 

 are generally cut up into many narrow segments. In many 

 of the BanuiiciiU, a submerged leaf niay be much divided, 

 whereas a leaf growing in air on the same plant remains 

 undivided. In many water plants, the ratio of leaf surface to 

 leaf bulk, is at least a hundred times greater than it is in 

 certain xerophytes. Taken altogether, the structure of a 

 submerged plant is simple, as compared with one of the same 

 order, gr(jwing on land. 



In most cases, the flower stem is elevated above the water 

 before flowering commences. In Limosella, which lives in 

 rain pools, if the plant should happen to be submerged at the 

 time of flowering, the flowers become cleistogamic {v. Viola, 

 p. 264) and self-pollinated. The perianth is, of course, water- 

 tight, and so the pollination takes place in air. Plants, the 

 poUination of which takes place in water are extremely rare. 

 Amongst them, however, may be mentioned the grass-wrack, 

 {Zoster a), which may be found througliout the colony, on the 

 mud-flats of the sea-shore. However, in Zostera, the pollen 

 grain has a unique structure. 



In MyriophijUum, the flowers are raised to the surface of 

 the water. Most of the species are nKinoscious, and, as in all 



