68 GROWTH AND SECRETIONS. 



of the preceding year remains ;f their vitality is 

 excited, and towards the end of winter radical 

 fibres are formed ; these, being fresh and vigor- 

 ous, begin to act, and pump up moisture from 

 the soil : thus, the revival of vegetation is effected 

 by the concurrence of two causes the activity 

 of the roots, and of the cellular envelope. The 

 sap arriving at the leafy parts* promotes the 

 development of the buds ; it first reaches those 

 at the summit of the branches, either because it 

 moves more readily in a vertical than in a lateral 

 direction, or because the wood and bark of the 

 extremity of the branches, being young and 

 herbaceous, the cells have there retained a 

 stronger vital action. When the action of the 

 leaves has furnished a certain quantity of nutri- 

 tive juice, it descends through the laticiferous 

 tissue, supplies the material from which the 

 tissues and secretions of the plant are formed, 

 and which " being poured out between the bark 

 and the newest layer of wood, is the viscid sub- 



t If a had preceding year has rendered the quantity of 

 nourishment small, the vegetation of spring is propor- 

 tionally weak. 



* If the sap, as it rises, finds any fissure in the wood, 

 it flows from it as from a fountain, as may be observed in 

 what are called the tears of the vine when pruned. 



