I 68 PLANT LIFE. 



hottest time in the afternoon. Spring is of course 

 the most vigorous period. The actual causes of the 

 ascent of water in the stem are generally supposed to 

 be : ( I ) The Osmosis or absorption of the root hairs. 

 This is partly osmosis (in the physical sense), and 

 partly due to the living action of the root cells. 



(2) The character of water itself which has a certain 

 cohesion, which must come into action in the capillary 

 passages of tracheids and vessels. 



(3) The transpiration of the leaf-cells through the 

 stomata or the lenticels of the cork sheet. This is partly 

 mere evaporation from the spongy and other cells in 

 the leaf, but is also due to their living action or control. 



In the writer's opinion, it is also probable that the 

 living cells of the stem assist in the raising of the water ; 

 but this is expressly denied by some experimentalists. 



It will be seen, that the facts known as to the 

 tensions existing in the stem do not present any 

 difficulty in understanding even such extraordinary 

 growths as are shown in the following cases. - A 

 Eucalyptus has grown from a mere seedling to 45 feet 

 in three years. This is of course a case of tropical 

 exuberance. 



The tallest trees appear to be the following : 



Eucalyptus amygdaloides, - 140-152 metres 459-498 feet. 



Mammoth Trees of California, 



Silver Fir, - - - - 



Spruce Fir, 



Pinus Sylvestris, Larch, and 



Cypress, - - - 48-53 „ 1 57-173 ,, 



The oak, ash, and other dicotyledons are seldom more 

 than 33 metres or about 100 feet. 



In America certain herbaceous stems, also, are said 

 to reach a great height. Wild Sunflowers 1 7 feet high, 



