STRUCTURE OF PLANTS. 



21 



made round the stem in a similar situation, the part of the stem above the 

 wound, or ligature, swells and increases in thickness, while that below it does 

 not ; a proof that in exogenous plants, the matter by which stems are thick- 

 ened, descends. Hence, when a shoot is cut through immediately below a 

 leaf-bud, the portion of the shoot left, dies back to the next bud. Hence, 

 also, has arisen the technical expression of " cut to the bud ;" which means, 

 that in pruning or cutting off a shoot, the section should be made so close 

 to a bud as that the wound may soon be healed over, and no stump left, 

 as is the case in gardens where trees have been carelessly pruned. The 

 greater the number of leaves on a shoot, or of leaf-buds on a stem or branch, 

 the greater will be the diameter of the parts below the leaves, buds, or 

 branches, and the contrary. 



84. Stems are either exogenous, growing from the outside ; endogenous, 

 growing from the interior ; or acrogenous, growing by elongation or dila- 

 tion, and mostly without buds. Exogenous stems consist of the pith, a 

 fungus-like matter, occupying the small cylindric space in the centre of the 

 stem, and never increasing in diameter ; of the medullary sheath, consisting 

 of a thin cylinder of spiral vessels and ducts, immediately surrounding the 

 pith j and of the wood, which surrounds the medullary sheath, in the form 

 of concentric layers, which layers are penetrated by projections from the 

 pith called medullary rays. In general every concentric layer requires a 

 year for its production ; and hence the age of a tree may be known by the 

 number of rings shown in the section of the main stem. In woody stems of 

 several years' growth, the interior of the wood is rendered hard by the 

 deposition of secreted matter, and is called heartwood ; while the more 

 recent exterior layers are known as soft wood or alburnum. 



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

 layers, one being added yearly on the inside, between the previously ex- 

 isting bark and the alburnum. Every layer of bark consists of woody fibre, 

 and ducts covered with parenchymous matter, the two former constituting 

 the liber, or inner bark, and the latter the cellular integument, epidermis, 

 or outer bark. The uses of the bark are to protect the alburnum, to serve 

 as a channel for the descending sap, and sometimes as a medium for the 

 deposition of the peculiar properties of plants. 



86. The medullary rays or plates consist of compressed vertical parallelo- 

 grams of cellular tissue, which connect together the different layers of wood, 

 and serve, at least in trees that are without dead wood in the centre of their 

 stems, as a communication between the pith and the bark. Between the 

 liber and the alburnum, a viscid secretion is found in spring, which renders 

 trees easily disbarked at that season, and this secretion is called cambium. 

 It has been supposed to nourish the descending fibres of the buds, and to 

 originate medullary rays. 



87. Endogenous plants have stems, which offer no distinction of pith, 

 medullary rays, wood, and bark ; the whole structure being composed of 

 bundles of vascular tissue among a mass of cellular tissue, surrounded by a 

 zone of cellular tissue and woody fibre : but as this exterior zone is not sepa- 

 rable from what it encloses by any natural division, it is consequently not bark. 

 Endogenous stems increase by the successive descent of new bundles of 

 vascular tissue into the cellular tissue towards the centre of the stem, and 

 these bundles of tissue gradually distend those previously formed, by 

 which means the diameter of the stem is slowly increased in thickness, and 

 its circumference in hardness. After this hardness has reached a certain 



