THE STEM. 165 



vary with the species, and with the season. The Ash, 

 has cinder-coloured twigs and black buds ; the Beech, 

 yellow or olive brown twigs, very different from the 

 white of the old trunks; the Sycamore, greenish-brown 

 twigs ; the Sloe, Elm, Alder, Rowan, and Birch, show twigs 

 with various red-brown or purplish tints ; Willow twigs 

 are yellow-green, but in some species may be purple ; 

 and the Dogwood is a bright warm-red colour. Probably 

 these tints are due to the joint effect of tannins, green 

 chlorophyll, the protective or warming red of anthocyanin, 

 (see p. 152) and cork. That the change is connected 

 with the chemical transformations necessary for the food- 

 stores, seems probable from the distinct series of colours 

 through which Birch twigs pass as spring approaches. 

 Sometimes, the food-stores are even utilised by man, as 

 in the case of maple sugar of Canada, which is regularly 

 drawn from the trees at the moment when it is fluid, 

 and not yet utilised by the buds. The " plant springs " 

 {Phytocrenes) are also tapped for water, and the sugary 

 fluid or ^^ pulque " of Agave Americana is much used in 

 Mexico and other places, where it is said 50 kilos, of sap 

 may be obtained from one plant. On the other hand, 

 the sugar obtained from the Sugar-cane, that got from 

 the various " Jaggery " Palms in India, and the starch of 

 the Sago Palms and Cycads, is probably a food-store 

 intended to help the plant when flowering. Sugars 

 and starches are accumulated, in the case of the Sago 

 Palm, for seven to eight years and dissipated in forming 

 the flowers and inflorescences. That there is plenty 

 of food material in certain stems, is proved by the 

 manner in which small cuttings of Willow branches take 

 root, and produce leaves. This peculiarity greatly 

 assists in distributing willow shrubs along river-sides. 

 A prostrate log, of almost any species, often throws up a 



