120 VEGETABLE SAP. 



Like other fluids the sap of trees flows quickest in 

 right lines. This is well known to the pruner and 

 trainer; who, by curving- or reversing- the leading 

 shoots, checks their luxuriance. It flows, however, 

 with greater celerity perpendicularly than in any 

 other direction. 



While the sap is stagnant in the branches, it is also 

 so in the roots, but with this difference, that in the 

 latter it is always fluid, owing to the higher tempera- 

 ture of the soil, and consequently, is ever ready to rise 

 as soon as the channels in the stem are open. In mild 

 winters (perhaps in all winters) there are signs of life 

 visible in the slowly swelling buds, and exsertion of 

 new fibres from the roots. 



The sap has been supposed to be transmutable into 

 cambium, and ultimately into wood ; but this requires 

 confirmation ; because, if sap be exuded from a 

 wound, it takes either the appearance of gum, as on 

 the cherry, or sanies, as on the elm ; but, when cam- 

 bium is protruded into air, it immediately assumes a 

 ligneous character. Besides, if sap be capable of 

 transmutation so as to be indurated by chemical 

 agency into timber, why is it not seen to be so changed 

 in the interior of stems, where it is found hardened 

 indeed, but perfectly homogeneous and free from 

 every sign of organisation ? It is said that elaborated 

 sap descends from the leaves down the bark, and is 

 thence attracted inwards to the centre of the tree; 

 but this being an invisible process and caused by 

 agents unknown to, and out of the reach of, mere 



