500 DEPENDENCE OF PLANT FORM ON SOIL AND CLIMATE. 



complex conditions arise which render the estimation of the soil's capacity for 

 retaining water very difficult. If permeable sandy soil, poor in humus, is deprived 

 of ground water and is dependent for its moisture solely on the atmosphere, the 

 plants growing in it will be retarded in their development if rain and dew are 

 absent for any length of time, and their outward appearance will be altered by this 

 restriction of growth. Annual plants subjected to a lack of moisture in the soil, 

 just at the time when their growth should be at its maximum, show best how far 

 these alterations will go. The stem-structures remain short, the foliage-leaves 

 shrink to their smallest extent, and no lateral shoots are developed. Only a few, 

 or perhaps only one, of the flower- buds mature; it is small, opens comparatively 

 very early, and the whole plant has a dwarfed aspect. Annual plants of the Poppy 

 (Papaver Rhceas, somniferum), Pheasant's Eye {Adonis cestivalis, flammea), Coxn- 

 cock\&{Agrostem'ma Githago), Corn&ower (Centaur ea Gy anus), and common Groundsel 

 (Senecio vulgaris) grown on a dry soil differ from plants grown in the same place, 

 but in a damp year, to such an extent in the size of all their parts that at first sight 

 they might be mistaken for other species. A clay soil which retains water is less 

 exposed to danger of too great drj^ness, but if it is not mixed with humus, and 

 therefore loosened, it has the disadvantage that the water it contains cannot take 

 up the inorganic foods quickly enough and in sufiicient quantity for the require- 

 ments of the plants. This drawback explains the surprising fact that plants grown 

 on heavy wet clay soils have a dwarfed appearance exactly like plants growing on 

 dry sandy soil. In regions liable to flooding by streams and rivers where not 

 infrequently sandy and clay soils, in all degrees of porosity and admixed with 

 humus in all possible proportions, are to be met with within a few yards of one 

 another, certain species of plants are to be found growing near together in all imagin- 

 able degrees of size, e.g. Aster Tripolium, Bidens cernua and tripartita, Polygonum 

 lapathifolium, Rumex maritimus, Veronica Anagallis. In places where the 

 seedlings cannot find enough free mineral foods, in spite of the abundant moisture 

 in the soil, the stem rises to some 3-8 cm.; in places which favour the absorption of 

 food, to some 50-80 cm. We will describe only one species, Veronica Anagallis, 

 more in detail. Plants of this species are found with stems 3-5 cm. high and 0'5 

 mm. thick, with foliage-leaves 6-12 mm. long and 5-6 mm. broad when fully 

 developed. The number of flowers in one inflorescence is about 4-5, the calyx and 

 ripe capsular fruit measure 3 mm. in length. Contrasting with these are plants 

 with stem 30-50 cm. high and 7-8 mm. thick, whose fully-formed leaves are 80 mm. 

 long and 35 mm. broad. There are 40-50 flowers in each inflorescence, and the 

 calyx and ripe capsule measure 4-5 mm. in length. Generally speaking these 

 plants are about ten times as large as the others. If the soils which give rise to. 

 such surprising diflferences in size are examined it will be noticed that the dwarfed 

 specimens are rooted in a heavy soil devoid of humus, while the large luxuriant 

 plants flourish in a clay soil which is mixed with plenty of humus, and is 

 therefore very open. Obviously the plants could not obtain from the heavy clay 

 soil what they required for the structure of a vigorous plant, even although the 



