THE ORIGIN AND STRUCTURE OF AQUATIC FLOWERING PLANTS. 91 



A similar separation, coupled with dislocation, takes place in sub- 

 merged stems, such as the rhizome of a water-lily. The whole becomes 

 a mass of cellular tissue having numerous scattered fibro-vascular 

 bundles dispersed through it. It thus resembles the stem of a palm. 

 Comparing the rhizome of the Lotus (Nelumhium.) with that of the 

 water-lily of the same family, we seem to see " degi'ees of dispersion," 

 so to say, for the strands, though all separate, are more regularly 

 arranged concentrically in the former than in the latter. 



The aerial stems and petioles of several moisture-loving plants have 

 a similar structure though growing in air, as the flower-stem of Anemone 

 rivularis, Caltlia palustris, Podophyllum peltatum, Hydrastis canaden- 

 sis, and the leaf -stalks of the edible rhubarb. 



It may be added that the foraiative tissue, camhium, of a timber 

 tree is absent in the isolated strands, or at most only the merest traces 

 of it are left in Monocotyledons. 



The Degenerative Effects of Water upon Leaves. — Perhaps the 

 most obvious effect of wat^r is to be seen in the foliage. There are 

 two principal types of submerged leaves, the dissected and the ribbon- 

 like. When a land dicotyledonous plant, with a netted- veined, fibro- 

 vascular system to the blade, becomes submerged, only the latter part 

 is developed, the intermediate tissue being arrested, as in the Water 

 Crowfoot. This is the commonest result. On the other hand, a long, 

 hnear form is the commonest among monocotyledonous water-plants. 

 This is rarer in Dicotyledons, but it is seen in the Mare's-tail (Hippuris), 

 the Av/l-worb (Suhularia), Shore- weed {Littorella), and Water Lobelia 

 {Lobelia Dortmanna). A similar form is found in the cryptogamous 

 plant, the Quillwort {Isoetes lacustris). 



This linear type of submerged leaf usually differs from a similar 

 one so common in Monocotyledons in that, being a degraded form of a 

 pinnately nerved blade, the lateral veins arise from the basal part of 

 the midrib and then run more or less parallel to the other end, 

 whereas in Monocotyledons with parallel venation they all commence 

 parallel by having issued from the stem separately, and continue so 

 to the apex. Moreover, in a dicotyledonous leaf the lateral veins are 

 offshoots from the midrib, but in Monocotyledons they are isolated 

 throughout, diverging right and left till one or a few only reach the 

 absolute apex. 



Occasionally the submerged leaf remains complete, but is very thin, 

 as of the White Water-lily and the Great Spearwort [Ranunculus 

 Lingua). 



Comparing submerged leaves with those of the same plant in air, 

 and seeing that the same dissected form is common to a very great 

 number of leaves of Dicotyledons when submerged, we are quite justi- 

 fied in concluding that water is the cause of the arrest of the tissues 

 in the latter; but it has been proved experimentally that it is so. 

 MacCallum, in the United States, selected a plant called Prosperpinaca 

 palustris, of the same family as our Mare's-tail. It has a lanceolate 

 leaf with a pinnate venation. When growing in water with the upper 



