553 



under side of the branch a])pears dark l)ecause of the dense formation of 

 red wood. We perceive in the anatomical pictures in Figs. ii8 and 119 

 the different structure of the elements of the ''red wood" and "strain wood." 



We obtain from R. Hartig reports, well worth considering, as to the 

 production of such differences. He states that, for example, in trunks with 

 an eccentric growth, the formation of the annual rings is especially strongly 

 developed on the branched side. The formation of red wood is proved to 

 be often dependent upon the prevalent direction of the wind, since the side 

 away from the wind favors red wood formation. If the west wind, for 

 example, strikes a spruce constantly, the west side will be strained. The 

 east side, toward which the tree is bent, is more strongly pressed and incited 

 to a stronger formation of red wood while the windy side, stretched by the 

 bending of the trunk, produces strain wood. Every branch will show just 

 such differences, for the weight of the needles will bend it downward. Its 

 morphologically upper side, therefore, is under a constant strain which 

 exercises a stimulus on the cambium. This consequently forms thinner- 

 walled,, less woody but longer tracheids and these represent the "strain 

 wood." 



Besides the action of the wind, the formation of the wood on every 

 branch is influenced by its surroundings. Shade from other trees, or prox- 

 imity to cliff's or walls, the one-sided eft'ect of greater moisture, partial 

 defoliation due to the grazing of animals, or other one-sided changes in the 

 nutrition of the branch will call forth a lack of uniformity in the quantity 

 and quality of the annual ring. From this it is clear that, in the action of 

 cold, the contraction of the tissue must vary greatly and the depression of 

 the branches must be very manifold, according to the distribution of strain 

 and red wood. Therefore, the observations made by diff'erent investigators 

 can have no general significance but should only be registered for the 

 present as individual cases. 



We will discuss thoroughly the differences due to strain in the section 

 on "Internal Cleavage." 



Freezing Back of Older Branch Tips. 



A freezing back of the branch tips is found in some of our woody 

 plants, almost as regularly as defoliation. Mulberry trees, acacias and 

 raspberries furnish the commonest exam])les of this. We owe more exact 

 studies on this point to v. MohP, who refers to the different stages in which 

 our woody plants are found at the beginning of the winter. 



In many 'plants the grov/th of the branches continues undisturbed so 

 long as the conditions are at all favorable for further development. This 

 growth only stands still during the period of cold and, as soon as the tem- 

 perature allows, begins again immediately at the point where it stopped in 

 the autumn. This is the case in the ivy (Hedera Helix) and the Savin 



1 Bot. Zeitung- 1848, p. 6. 



