36 THE BIOLOGY OF FLOWERING PLANTS 



survey of conditions in the Algerian Sahara, describes many 

 extensive root systems ; in plants growing on the deeper 

 soils of the desert, deeply penetrating tap roots are met with. 

 Volkens (1887), too, speaks of the shrubs of the Egyptian 

 desert as characterised by very deep -going roots which 

 tap deep-lying moist soil and water veins. " Some are 

 probably to be found only in places where well-fed veins 

 of water are present, so the widespread colocinth." Roots 

 of Acacias penetrating to ground water at a depth of 40 ft. 

 were observed during the excavation of the Suez Canal. 

 Later investigators regard this arrangement as exceptional 

 for desert plants. 



Other Root Systems. — The root systems of the native 

 and crop plants of Britain have been little studied ; this is 

 practically a virgin field for investigation. Some observa- 

 tions have been made on the regions of the soil exploited 

 by the systems of diflPerent plants living together. 



A certain amount of information exists on the root 

 systems of aquatic and marsh plants. In Kirchner and in 

 Sherff (1912), the types of system for a number of mono- 

 cotyledons and for the plants of an American marsh may 

 be inferred from the figures. A short summary is given by 

 Arber, whose account deals more with mechanical features. 



As might be expected, the plant growing in water or 

 mud has in general a reduced root system ; the absorbing 

 surface need not be so great, and moreover the presence of 

 water may reduce very considerably the necessity for 

 resisting strains. In Potamogeton, both in species with 

 floating leaves and in those entirely submerged, bunches of 

 stout unbranched adventitious roots, a few inches long, 

 arise from the nodes. Castalia produces abundant adventi- 

 tious roots which may reach a foot in length and are sparingly 

 beset with fine rootlets. Sagittaria and Alisma, sending 

 leaves and shoots above the surface of the water, have many 

 short stout roots sometimes with fine rootlets and sometimes 

 without. Plants like Ranunculus Flammula, which grow in 

 very wet soil, have systems of matted fibrous roots with 

 numerous fine rootlets attaining the length of a foot ; while 



