14 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



angles to the parent root, they simply continue to elongate more or less 

 in the same direction. If, however, they be in a comparatively drier 

 place and a moister one be at some distance, they will grow in the 

 direction of the latter quite irrespective of gravitation. So that Hydro- 

 tropism easily overcomes Helkotropism. 



A good illustration of what may be most probably due to gravitation 

 alone is seen in the descending aerial roots of the Indian fig or banyan ; 

 inasmuch as the dense shade would be above and light below ; so that as 

 the roots pass through the air this would be throughout equally dry or 

 moist as the case may be, leaving gravitation alone to exert its influence. 



Thermotropism. 



Another stimulus of attraction causing movements, is heat. When 

 one side of a plant receives a higher degree of temperature than the 

 other, stems and leaves will often turn to the warmer side. Kerner 

 observes that in the high Alps, the air being rarefied, the exposed ground 

 receives great heat from the sun ; but the temperature of the air above it 

 may be lower ; so that many plants creep along the ground, such as 

 species of willow, " Jumperns pyramidata var. humilis, called the ' creep- 

 ing Sabina.' J. nana or alpina, i.e. a dwarf form of J. communis, sends 

 out from the collar of the root long branches trailing on the ground, as 

 is generally the case with Alpine plants."* 



Kerner gives the excess of the mean temperature of the soil over that 

 of the air of different altitudes on the Central Tyrolese Alps as follows : 

 at 1,000 m., 1-5°C. ; at 1,300 m., 17°C. ; at 1,600 m., 2-4°C. ; at 1,900 m., 

 3-0°C. ; and at 2,200 m., 3-6°C. 



In warmer climates similar phenomena are to be seen. I frequently 

 noticed in Malta that very many plants are prostrate when growing by 

 road sides, where the limestone rock is often exposed and gets heated. 

 The Maltese ' clover,' Hedysarum coronaria, grows to a height of 3 feet 

 in the fields, but stray plants are perfectly prostrate by the roads. The 

 same thing occurs with both Malva sylvestris and M. parviflora. 



Similarly with leaves, a small species of dandelion often grows 

 between the loose stones of walls in Malta. When extracted, the leaves, 

 which were flat against the hot vertical wall, curled back against the root,, 

 and refused to be straightened out when required to be dried. 



In early spring in England, bluebells may be seen having their first 

 formed leaves lying flat on the ground, and daisies and plantains always 

 have them so in a close- cut lawn. 



The following observations on temperatures will perhaps show how 

 thermotropism may account for the prostrate position independently or 

 conjointly with light, a feature of common occurrence in this country. 



The temperature at the surface of the damp soil by the side of a blue- 

 bell growing in the shade of trees, at 9.45 a.m., April 15, 1891, a sunless 

 morning, was 47° F. The temperature of the air three inches above the 

 ground at the same time and place was 44*5°. At 4 p.m. of the same day, 

 on the surface of the soil the temperature was 60° ; while that of the air 



* See Origin of Plant Structures, pp. 101, 102. 



