142 



BOTANY 



PART I 



than in the following seasons (Figs. 151 yi, 157). For this reason a 

 difference is perceptible between the early wood (spring wood), 

 which is composed of large elements especially active in the convey- 

 ance of water (Fig. 151 /), and the LATE wood (autumn wood), 

 consisting of narrow elements which impart to a stem its necessar)'^ 



rigidity (Fig. 151 s). Throughout 

 the greater part of the temperate 

 zone, the formation of wood ceases 

 in the latter part of August until 

 the following spring, when the 

 larger elements of the spring wood 

 are again developed. Owing to the 

 contrast in the structure of the 

 spring and the autumn wood, the 

 limits(Fig. 149 i) between successive 

 annual rings of growth become so 

 sharply defined as to be visible even 

 to the naked eye, and so serve as a 

 means of computing the age of a 

 plant. 



Under certain conditions the number 

 of annual rings may exceed tlie number 

 of years of growth, as, for instance, when 



MIDSUMMER GROWTH OCCUrs, SUCh aS 



commonly happens in the Oak, when, 

 after the destruction of leaves by cater- 

 pillars, a second formation of spring wood 

 is occasioned by the new outgrowths thus 

 induced. In the evergreen trees of our 

 latitude the vessels are not crowded to- 

 gether in the spring wood, but uniformly 

 distributed on the annual ring (^^^). In 

 the wood of tropical plants the annual 

 rings may be entirely absent. This occurs, 

 for example, in the tropical Conifers of 

 the genus Araucaria, which, in this re- 

 spect, show a marked contrast to the 

 Fig. 150.-Dia8Ta.u to illustrate the secondary Conifers of the northern zone. Any in- 

 growth in thicknes.s of the stem and :oot 



of a Gymnosperm or Dicotyledon, c, Cam- 

 bium, indicated by a dotted line ; m, pith ; 



terruption of growth, such as would occur 

 during a dry period, followed by a period 

 1,2, 3, 4, successive annual rings of wood; Z), of renewed activity, may occasion the 

 bast ; sw, limit between main stem and tap- forni^tiou of annual rings even in tropical 

 root. 



jjlants. 



Although a cessation in the formation of wood takes place so early, 

 the caiubiinn continues to form bast so long as climatic conditions 

 permit. As a rule, however, far fewer elements are added to the bast 

 than to the wood. Owing to the continued growth in length, the 

 higher a cross-section is made of a gymnospermous or dicotyledonous 



