Although most species readily fall into one or another ecotype, in some in- 

 stances individuals belonging to the same species, such as in Potamogeton nodosus 

 and Polygonum amphibium, may occur as submersed or floating aquatics to am- 

 phibious, riparian or strand plants rooted in mud. These plants usually, but not 

 always, differ somewhat in habit, in accordance to the ecotype in which they are 

 found. 



We have found, as other have found before us, that aquatic and wetland plants 

 are not always dependable and of long-endurance in their particular habitats. A 

 species abundant in one season may disappear for one or more seasons only to 

 reappear in a later season. There are various possibilities as to why these plants 

 behave thusly. A close approach to temporary extermination might result from 

 overfeeding by wildlife; a change in the ionic composition of the water might be 

 critical; especially in the case of vegetative reproduction, some water plants, as in 

 many orchids, might have a longer resting period than just one year; fluctuating 

 water levels might create physiological problems for the species. 



Analagous habitats may be found in entirely different parts of our region and 

 under an entirely different set of factors, but they may reveal certain similarities. 

 For instance, the Weches fossil formation near San Augustine, Texas that is seepy 

 and wet only in the spring long enough to support to maturity the annual crucifer, 

 Leavenworthia aurea, is comparable to tidal zones where the usually perennial 

 plants that occupy such zones must have a periodic replenishment of water in 

 order to thrive. Under both situations the water needs of the plant are met at 

 critical times to assure propagation and/ or continued survival. 



Vascular plants that are strictly aquatic, although relatively few in number 

 when compared to their dryland terrestrial relatives, offer a multitude of taxo- 

 nomic difficulties because of their diversity of habit and bewildering variations that 

 include heterophylly and peculiar modifications in sexual and vegetative repro- 

 duction. 



Heterophylly, the presence on a single individual of two or more distinct types 

 of leaves, in habit, shape and/ or anatomy is prominently displayed in many 

 aquatic species in such genera as Callitriche, Potamogeton, Sagittaria, Rammculus, 

 Cabomba, Myriophyllum, Proserpinaca, Alisma, Armoracia and Echinodonis. 

 This leaf difference in the same individual has created problems in the identifica- 

 tion of many species in the above, as well as other, genera. This unique charac- 

 teristic must be taken into consideration in dealing taxonomically with these 

 species. 



Peltate leaves, as in Nelumbo (Fig. 447), are considered to be the most 

 mechanically efficient of all types of floating leaves. Although the leaves of 

 Nymphoides (Fig. 1) do not have the thickness and toughness of such species as 

 in Nymphaea, (Fig. 442) and Nuphar (Fig. 443), they do demonstrate the 

 characteristic leaf of many floating leaved plants in their rounded blade and 

 entire margins. The wax bloom of the cuticle on the upper surface, as in all plants 

 with floating leaves, prevents excessive wetting of the leaf. The petioles that sup- 

 port these leaves are strong and pliable, and their buoyance and support are 

 further enhanced by air-filled lacunae on the lower surface which are centered 

 and more prominent along the midrib and near or about the petiole. 



Floating leaves, in being exposed to both air and water at the same time, are 

 rather unique and they have developed features that enable them to better with- 

 stand the hazards of their environment. 



Most of our emergent hydrophytes produce aerenchyma — a spongy tissue de- 

 veloped mostly on the stems and branches at or below water level. This tissue may 

 have several functions as for buoyancy for weak, arching stems of Decodon 

 verticillatus, for storage of oxygen or for insulation and protection. Development 

 of aerenchyma has not been noted by us in any emergent monocot species, even 



