17 



intermittently affect the functions of the plant ; hence he 

 accounted for the occurrence of two zones of tissue, 

 on each ray of annual growth, one consisting of the 

 ordinary tubes of wood-fibre with discs common to 

 all conifers, the other consisting of tubes with no 

 discs but covered with spiral striae giving the appear- 

 ance of each tube being formed by a twisted band. 

 The deflexion of the currents of the sea, from what- 

 ever cause, materially affects the climate of a 

 country coming under their influence ; had not the 

 gulf-stream for instance returned to our shores at the 

 close of the glacial epoch the temperature of Great 

 Britain would now be that of Labrador ; we should be 

 scarcely receiving any appreciable increase of heat 

 from the equatorial region by means of aerial currents, 

 for heated air rising from the equator as soon as it has 

 reached the intense cold of the upper regions soon 

 parts with its caloric. The warmth, therefore, which 

 the south-west winds bring us, is not derived from 

 equatorial zephyrs, but from the great oceanic current 

 which takes its rise in the southern ocean, and passing 

 on north of the equator, imparts its genial influence 

 beyond the boundary of the polar-circle. This current 

 is fifty miles broad and a thousand feet deep, flowing 

 at the rate of four miles an hour. The enormous 

 extent to which the heat of the earth is affected by 

 means of oceanic currents throws some light upon the 

 mystery of geological climates. There is no better 

 instance of climatal effect upon plant-life than the palm ; 

 which dwindles down to a dwarf shrub at its extreme 



