22 



INTRODUCTION. 



vascular tissue, which is called the medullary sheath ; it is composed of woody- 

 fibre and spiral vessels, and is the 

 Fig- 11. only part of the stem in which these 



latter occur. Both this sheath and 

 the wood are traversed by narrow 

 plates of condensed cellular tissue 

 passing from the pith to the bark, and 

 denominated medullary rays. The 

 number of zones or layers in a stem 

 is in proportion to its age, each 

 layer being the product of a single 

 year's growth, hence the exact pe- 

 riod of existence of the tree can be 

 correctly ascertained from them.* 

 The woody portion is also divided 

 into two parts ; one, exterior, new, 

 colourless, and permeable to the cir- 

 ic* cdcae culating fluids, called Alburnum or 



sections of an Exogen of 3 years old. ap ith. b me- ' Sap wood ; the other central, denser, 



dullary sheath, exterior to which are 3 layers of charged with certain Secretions, and 

 wood, each formed of c dotted ducts and d woody . ° , , t , . . . j t-» 



fibre e bark. impermeable ; this is termed Dura- 



men or Heart ivood. It is generally 

 of a darker colour, and as it does not assist in maintaining the functions 

 of the tree, it may decay, without any injury to the vitality of the plant. Each 

 layer of wood consists of vessels or ducts and woody fibre. The first of 



these is the earliest formed, and therefore nearest 

 the centre : the latter is not produced till a later 

 period. Their relative arrangement, however, is 

 not the same in different trees, and occasions the 

 numerous varieties in the texture met with in 

 woods. In the pine and fir tribe the structure is 

 so dissimilar from that of other exogens that some 

 botanists have thought that they constituted 

 another class to which the name of Gymnogens 

 has been given. There are no ducts in their 

 wood, whilst the diameter of the tubes of the wood 

 is greater than usual. Other remarkable devia- 

 tions also occur; thus in Calycanthus the old 

 Bark stems have four distinct axes of growth between 

 c Medullary rays, d Bundles ofthe regular wood and the bark. In the woody 



woody fibres. . . , y , . . , , J 



Artsto/ochias there are no regular zones or layers, 

 the wood consisting of wedges separated by large medullary rays and ar- 

 ranged around the central pith. 



The stem of an Endogen presents no distinct separation into pith, wood, 

 and bark. The woody portion consists of bundles of fibres distributed 

 throughout a cellular tissue, and not presenting any appearance of zones. 

 These fibrous bundles arise from the leaves and pass downwards towards 

 the interior part of the stem, and afterwards turn outwards and mingle 



Fig. 12. 



Exogenous stem, a Pith. 



* This is strictly applicable only to trees in temperate and cold climates, which change 

 their leaves but once every year. In tropical regions, there is reason to believe that more 

 than one layer is deposited annually. From some observations made in the West Indies 

 several years since, we were led to this view ofthe subject, and are glad to find that Dr. 

 Carpenter entertains the same views. (See Vcg. Physiology, 98.) 



