154 



BOTANY 



PART I 



injurious influences, the buds destined for the succeeding spring may unfold, and 

 the formation of the new foliage brings about a second formation of spring wood. 

 On the other hand, woody plants that usually have definite annual rings may 

 exceptionally show a smaller number of rings than that corresponding to their 

 age, owing to the limits between some of the rings not being clearly marked. In 

 this way the number of rings on one 

 radius of the stem may be less than 

 when they are counted on another 

 radius. 



The wood of the stems and 

 roots of Dicotyledons can be 

 readily distinguished from that 

 of a Gymnosperm even when 

 only slightly magnified (Figs. 



FIG. 178. Transverse section of a stem of 

 Tilia ulmifolia, in the fourth year of its 

 growth. pr,iPrimary cortex ; c, cambium 

 ring ; cr, bast ; pm, primary medullary 

 ray ; pm', expanded extremity of a primary 

 medullary ray ; sm, secondary medullary 

 ray ; g, limit of third year's wood. ( x 6. 

 After SCHENCK.) 



FIG. 179. Portion of a four-year-old stem of the 

 Pine, Pinus sylvestris, cut in winter, q, Transverse 

 view ; I, radial view ; t, tangential view ; /, spring 

 wood ; s, autumn wood ; m, medulla ; p, proto- 

 xylem ; 1, 2, 3, k, the four successive annual rings 

 of the wood ; i, junction of the wood of successive 

 years ; ms', ms"', ms, medullary rays in trans- 

 verse, radial, and tangential view; ms", radial 

 view of medullary rays in the bast ; c, cambium 

 ring ; b, bast ; h, resin canals ; br, bark, external 

 to the first periderm layer, and formed from 

 the primary cortex. ( x 6. After SCHENCK.) 



180, 181, 182). Not only are wood-fibres and usually wide tracheae 

 present, in addition to tracheides and parenchyma, but the unequal 

 growth of the various component elements leads to a departure from 

 their original radial arrangement. In the spring wood there are 

 numbers of very wide vessels (Figs. 180, 181 m), while narrow 

 wood-fibres (/) and fibre tracheides (t) predominate in the autumn 

 wood. 



In some Dicotyledons the annual rings are not distinct because the various 

 elements of the wood are nearly uniformly distributed in the season's growth. 

 This is the case in the Willow, and in the Wild Vine it may be impossible to count 

 the rings. In the woody plants of tropical regions, when there is no seasonal 



