EVOLUTION^BY DEGENERATION. 21 



effect of an excess of water upon all the parts of an aquatic plant 

 which grow submerged. 



If the seeds of the Water-Crowfoot be grown in a garden soil, 

 or in the mud when a pond dries up, they all germinate and grow up 

 into vigorous land-plants ; but the whole internal structure responds 

 to aerial conditions and is developed in adaptation to them, exactly 

 as an ordinary land-plant does. Yet the leaf retains the dissected 

 type, proving that this " acquired character " has become fixed and 

 is hereditary. 



Another effect of water is to arrest the development of the first 

 or axial root of any dicotyledonous plant germinating and growing 

 in it. The result is that as the stem grows it increases in diameter 

 upwards till it forms an inverted cone, and then secondary roots 

 are supplied from the joints ; so they look Hke tent-ropes supporting 

 it. This feature is characteristic of all members of the class 

 Monocotyledons, as well as all aquatic Dicotyledons, so the conclusion 

 follows (based, however, on a great number of other coincidences as 

 well) that the former Class was descended from aquatic dicotyledons ; 

 but it must have been at a very early period, for no monocotyledon 

 has submerged, finely divided leaves ; all they produce is a flat petiole 

 giving the ribbon-type as seen in all grasses, sedges, &c., with the 

 *' veins " running more or less parallel from one end to the other. 



If they produce a blade, it follows at the end of the flat petiole, 

 when it reaches the surface and floats, or rises into the air as in the 

 Arrowhead. 



Water may equally affect the flowers. Thus the Mare's-tail family 

 contains genera with very degraded flowers. The Starwort [Callitriche) 

 has only a stamen for a male flower and a pistil for a female ; while 

 the Sea-wrack (Zostera) is in the same degraded condition. 



Many aquatic dicotyledons are dwarfed, with very narrow leaves 

 and imperfect flowers, e.g. Water-blinks of the Loose-strife family ; 

 the Shore-weed, aUied to the Plantains ; the Mudwort, to the Foxglove ; 

 the Marsh Samphire, to the Goosefoots, &c. ; while the Duckweeds 

 are most degraded of all. There is no distinction between leaf and 

 stem, and the male and female flowers consist of the stamen and one 

 carpel respectively. Botanists regard them as degenerate affinities 

 of the Arum family. 



Degradations through Drought. — Let us now see the results of 

 impoverishment. Many little annuals live habitually in dry waste 

 ground or in very gravelly or sandy soil. The common effect of 

 semi-starvation is to reduce the size of the whole plant, and of each 

 of its organs. The following are a few examples from various families. 

 The Vernal Whitlow-grass, abundant in walls, paths, &c., with its 

 leaves half-an-inch long in a rosette, and small, white petals, belongs 

 to the Crucifers ; the Pink family includes the very small tufted Pearl- 

 worts, as Sagina apetala (4-10 inches), so called because it has minute 

 petals or none at all. Like the preceding, it is found on wall-tops &c. 

 The All-seed, Radiola linoides (1-4 inches) frequents sandy ground. 



