35] 



CHAPTER II. THE TISSUES. 



141 



Both the woody fibres and the thick-walled fibrous cells may 

 eventually become chambered by the formation of delicate 

 transverse septa. 



The structure of the secondary wood of the root is, in 

 some cases (e.g. Conifers), almost identical with 

 that of the corresponding stem ; this is the case, 

 to a somewhat less degree, in woody Dicotyledons ; 

 whilst in herbaceous Dicotyledons the structure 

 may be very different in the two members, owing, 



FIG. 111. Isolated constituents of the secondary wood of the Lime (Tilia 

 parvifolia). A and B wood-fibres ; C wood-parenchyma ; D and E tracheids ; 

 F segment of a wood- vessel (trachea). G is a bast-fibre. ( x 180 : after Stras- 

 burger.) 



chiefly, to the development of more medullary ray, but 

 less woody tissue, in the root (see above p. 139). 



A transverse section of a stem or a root of most coni- 

 ferous or dicotyledonous trees or shrubs exhibits, even 

 to the naked eye, a series of concentric layers in the 

 secondary wood known as the annual rings (Fig. 112). 

 These layers result from the fact that the wood formed 

 in the spring is differently constituted from that which is formed 



