678 SECONDARY GROWTH IN THICKNESS 



on the upper, and 6*5 cm. on the lower side). The thirteenth annual 

 ring initiates the subsequent prodigious development of the upper half ; 

 this particular ring is 40 mm. thick above, 5 mm. on the flanks of the 

 root, and not more than 2 "5 mm. below. The greatest thicknesses 

 attained by individual rings on the upper side are 60 mm. (nineteenth 

 ring) and 57 mm. (fifteenth ring). It will be noticed that a certain 

 amount of hyponastic thickening also takes place from the fifteenth ring 

 onwards, though this is always insignificant in comparison with the 

 permanent and extensive epinastic tendency, and is most probably 

 not a normal feature at all. It is further impossible, in the case of 

 this particular cross-section, to determine with any certainty, whether 

 all the annual rings are complete or whether some of them die out 

 altogether on the lower side. Ursprung has explained why it is an 

 advantage for plank-roots to be epinastic rather than hyponastic. In 

 the first place this type of growth is unaffected by the pressure of the 

 soil ; and, secondly, the buttressing effect of the plank-roots is greater 

 if the latter are situated above ground. 



The successive annual rings of a trunk or branch are more or less 

 clearly distinguishable in a transverse section, because of the anatomical 

 difference between spring and autumn wood. This difference may be 

 due to an unequal distribution of the various elements within each 

 ring, or it may depend upon differences in the shape and structure of 

 homologous elements in the two regions of the ring ; very often the 

 two factors act in conjunction with one another. 



As a rule, spring wood contains a larger proportion of vessels and 

 tracheides than autumn wood ; the vessels are also usually much wider 

 in the former type of wood, which hence has a looser and more porous 

 texture. The elements of the spring wood are, moreover, as a rule, 

 relatively thin-walled, the contrast in this respect to the cells of the 

 autumn wood being accentuated by the fact that the latter are most 

 often tangentially flattened to a very considerable extent (Figs. 281, 

 282). The aforesaid differences are most striking, where the com- 

 position of the wood is very homogeneous, as it is among Conifers. 

 Thus, Von Mohl found the average radial diameter of the fibrous 

 tracheides, in a well-grown 30-year-old trunk of Pinus sylvcstris, to 

 be from three to six times as great in the spring wood as it is in the 

 autumn wood. The flattening of the autumnal tracheides, measured 

 by the ratio between the radial and the tangential diameters, amounted 

 to r, while their walls were 1"6 times as thick as those of the spring 

 tracheides. The tracheides thus primarily represent mechanical cells 

 in the autumn wood, and conducting elements in the spring wood. 



There are a variety of histological features, which help to secure 

 radial communication between the conducting elements of successive 



