7<D LAND-FORMING PLANTS. 



Other plants with similar rhizomes grow outwards from the firm 

 land of the shore above the main shoots of Phragmites. Comarum 

 palustre, Menyanthes trifoliata, Scirpus lacustris, and Hippuris are 

 good examples. The water is thus divided by a network of 

 horizontal runners and erect stems and leaf-stalks, and in this 

 entanglement, mud, algae, dead leaves, etc., rapidly increase. 



On the surface amongst these stems, Plankton-forms such as 

 the Lemnas, Anabaina flosaquae (Hogganfield Loch), and other 

 algae and diatoms increase to an enormous extent, chiefly because 

 they are wind-sheltered. Shallow water is thus rapidly encroached 

 upon, and small ponds may become choked up in a few years. 

 If, however, the bottom is steep, the land-formation is much 

 more slow. No doubt, the perpetual falling off of mud will 

 gradually raise the level, but the march is imperceptible. 



Another important land-former is Glyceria fluitans var. /J. 

 This grass is very active on the Forth and Clyde Canal on the 

 upper side of the locks where a sort of bay is formed as the Canal 

 narrows to the lock entrance. It sends out floating rhizomes 

 sometimes 5 feet long or more. Besides diatoms and floating 

 debris many Spirogyras, Zygnemas, etc., grow epiphytically upon 

 it. As the land accumulates (one can distinctly trace in places 3 

 feet of land acquired by it) Poa annua and many mud-plants press 

 in amongst its big swollen rhizomes and soon kill these out and 

 occupy their place. All these plants, Scirpus lacustris, S. maritimus, 

 S. rufus, Phragmites, Glyceria fluitans, as well as Hippuris and 

 Menyanthes, agree in certain points. They have long internodes, 

 a very loose spongy structure, and the rhizome is horizontal. The 

 long internodes are a special case of the law that plant-parts in 

 water tend to elongate. The sponginess has never been properly 

 explained. (My own view is that the epidermis is so weak, and 

 the whole plant so much without hard substance, that there is 

 nothing to check a widening of the tissue). As to the horizontal 

 habit of the rhizomes, it is partly dependent on light, for the 

 ends certainly turn up whenever they can manage to do so. 



These examples are sufficient to show how prevalent land- 

 formers are in this country. In the British Flora I count 26 

 genera which are land-formers of this type. Hippuris, Glyceria, 

 Phragmites, Digraphis (riversides) and all the genera of Cyperaceae 

 have representatives of this kind ; others are Juncus, Butomus, 



