1918.] T HE TWIST OF WOOD-FIBER IN THE TAMRAC PINE 65 



spaced equally at an interval such that only after making eight 

 complete turns around the stem, and passing twenty-one stations, 

 do we come to a station, No. 22, which stands directly above No. 1 

 and begins a second cycle exactly like the first. This particular ar- 

 rangement is therefore called a right-hand -^ T arrangement. The 

 corresponding left-hand arrangement would be exactly the same, 

 save that right and left would everywhere change places, as they 

 would were we to look at the reflection of this diagram in a mirror. 

 Now the general facts regarding the growth of wood-fiber are 

 well understood, and may be summarized as follows: — The leaves 

 develop one after another in ascending order according to their 

 positions on the fundamental spiral. From the base of each, at a cer- 

 tain stage of development, bundles of wood-fibers begin to form, 

 and these extend downward along the axis, interlacing with similar 

 bundles from other leaves, till finally they form together a continu- 

 ous shell of wood just within the layer of bark. Presumably the 

 growth might be directly downward were there no obstacles in the 

 way. But obstacles there are, as a glance at the diagram will show. 

 Every leaf -base is such an obstacle standing more or less in the way 

 of growth from above. For, if we choose as a starting point one of 

 the upper leaf-bases in the diagram— say No. 23 — we see that the 

 space below it is free no farther than to No. 15. The left-hand mar- 

 gin of the bundle of wood-fibers from No. 23 is sure to encounter 

 the right shoulder of No. 15, where doubtless some of its fibers will 

 lose themselves in effecting the junction ; and from this point on the 

 bundle from No. 15, already established, forms a fixed barrier to any 

 further spread of the bundle from No. 23 in that direction. But the 

 fibers from No. 23 are still in process of formation, and are free to 

 develop along the line of least resistance. The growth from No. 23 

 therefore we may think of as crowded over to the right until its 

 edge touches the edge of the bundle from No. 18. Confined then be- 

 tween these two barriers, its general direction may be taken to be 

 the line between No. 23 and No. 10 — at which latter point its sep- 

 arate identity may be considered as coming to an end. 



But what happens in the case of No. 23 is repeated in the case of 

 every other station in the whole scheme. The exact repetition of the 

 quincuncial pattern gives to every station in the diagram the very 

 same position with regard to its neighbors, the same fixed obstacles, 

 and the same open fairway. The whole sheet of wood-fiber, made up 

 of the bundles from each leaf-base, must have the same inclination 

 that they all have separately. Though the actual growth from each 

 station is downward, yet since the development as a whole is up- 

 ward, we shall avoid confusion by naming the direction of the slant 

 here shown as if its lines really grew upward. We call it therefore a 

 left-hand twist. 



The hypothesis here is that a definite pattern of leaf-arrange- 

 ment may cause a definite deflection in the line of growth of wood- 



