162 



ACADEMIC BOTANY. 



and the new wood, the tree will die, because deprived of its circula- 

 tion. 



398. Acrogens and Endogens have no cambium -layer ; 

 being without differentiation into pith, wood, and bark, 

 they do not need it. In annual exogenous stems, like the 

 Melon, it is not fully organized ; but in perennial ones it 

 becomes a complicated zone, communicating with the bark- 

 layers on the one hand, the wood-layers on the other. 



399. Digestion. — The pabulum absorbed by the fibrils 

 and roots is sent up to the leaves 

 through the circulation just de- 

 scribed. The leaves themselves 

 absorb or inhale food also from 

 the air; it is taken in through 

 tlie stomata (Fig. 234, s, s). The 

 fresh elements thus received en- 

 ter the parenchyma, or pulp-cells 

 (pi, ps), where they mingle with 

 the elements brought up by the 

 pabulum. Here all are subjected 

 to the action of sunlight and 

 chlorophyl, the substance which 



gives the green hue to leaves, and with which we became 

 acquainted in the first living cell (37). 



These two agents are the. chief factors in the work of digestion, 

 which takes place in the parenchyma alone, and in this onlj' when 

 acted upon by sunlight. After digestion, the juices are sent down as 

 Elaborated sap ; hut this sap does not pass through the vessels which 

 carried up the pabulum ; it goes through the bark-cells. As it passes 

 it is distributed wherever it is needed, from stem to root ; iirst on the 

 under-surface of the leaf; then in the leaf-stalk; then in the liber; 

 food in the young cells of incipient buds ; a rich supply in the cam- 

 bium ; wood to the wood, bark to the bark. The remainder is taken 

 down to the roots, where it lies dormant in cold climates through the 

 winter, to be used as the base of operations in the spring. Thus the 

 tree grows in height and size ; the old wood pushed to the centre be- 

 comes heartwood ; the old bark pushed to the circumference is split 

 and furrowed, forming surface-bark. 



400. Formed Material. — When the protoplasm has done 

 its work of absorption, circulation, digestion, and deposit, 

 it leaves the old cells aud passes on, forming new ones, in 

 which the same processes are carried forward. The com- 



FiG. 234.— Veit. eec. of Balaam If. 

 {ImptttienB Bahtamina) : es, epidermlB 

 of upper surface ; ei, of lower surface ; 

 8, n, stomata ; I, I, Incunes ; pi, paren- 

 chyma of lower surface ; ps, of 

 upper surface; X. 



