RUDIMENTS OP BOTANY. XI 



composed of cellular tissue, woody fibre and ducts, and are traversed 

 by the medullary rays composed of cellular tissue, and connecting the 

 centre with the circumference. The fully formed or central layers are 

 called the heart-wood, and the exterior the alburnum. 



23. The bark surrounds the wood, and like it consists of concentric 

 layers, but of these the hardest or most fully formed is exterior and 

 the youngest interior. Each concentric layer is composed of woody 

 fibre and ducts covered externally by a layer of cellular tissue ; the 

 woody fibre and ducts constituting the liber ; and the outer cellular 

 tissue, the epidermis. 



24. The cambium is a viscid secretion which is formed in the spring, 

 between the liber and alburnum. 



25. The stem of Endogenous plants presents no distinction of pith, 

 medullary rays, wood and bark, but is formed of bundles of ducts and 

 spiral vessels interspersed through a cellular tissue ; and this is sur- 

 rounded by a stratum of cellular tissue and woody fibre different from 

 bark, inasmuch as it cannot be separated from the stem itself. Such 

 plants have their diameter increased by the addition of central vascular 

 tissue and ducts. 



26. Projections from the medullary sheaths sometimes reach the 

 circumference of the stem and branches, forming what are called nodi, 

 to which are attached leaves and leaf buds, and the spaces between 

 these are called internodia. 



27. Whatever is produced by the evolution of a leaf bud is a branch : 

 A spine therefore is a kind of branch ; it differs from the prickle which 

 is a mere dilatation of the cellular portion of the bark. 



28. The stem peculiar to the grasses and other allied tribes is term- 

 ed a culm. ' This is simple or rarely branched, generally hollow within 

 or fistulose, and separated at intervals by knots ; or partitions from 

 which issue the leaves. 



29. The stem may be simple or branched, and with the branches 

 may be cylindrical, or conical ; round, (terete, ) or angled ; smootfi, fur- 

 rowed, or rough, or hairy, &c. 



30. With regard to duration the stem is 



a. Annual, (0) when it is completely developed and decays during 

 the same season. 



b. Biennial, ( $ ) when it produces fruit the second season and then 

 decays, 



c. Perennial, (If) when it produces flowers and fruit during many 

 successive seasons. 



31. The term herl or herbaceous employed in opposition to perennial, 

 denotes that the stem generally dies down to the ground every year. 



LEAF-BUDS. 



32. Buds are of two kinds, leaf-buds and flower-buds. 



33. Leaf -buds consist of rudimentary leaves surrounding a vital point, 

 the tissue of which is capable of elongation ; upwards in the form of 

 stem, and downwards in the form of wood or root. 



34. Flotcer-buds consist of rudimentary leaves surrounding a point, 

 which does not elongate after it is once developed, and assumes when 

 fully developed, the form of reproductive apparatus. 



