CHAPTER III 

 THE FIBRO VASCULAR TISSUES: WOOD GENERAL 



Since the fibrovascular tissues are on the whole the most impor- 

 tant in the organization of the higher plants both from the evolu- 

 tionary and from the physiological standpoint, it will be well to 

 begin with their anatomy. The most conspicuous and best- 

 developed portion of the fibrovascular system in land plants is 

 the wood. The aggregation of elements assembled under this 

 heading affords also the best exemplification of the process of 

 evolution in the higher plants as the result of the progressive 

 development of the principle of division of labor and the gradual 

 adaptations of plants to the more complicated conditions of life 

 obtaining in later geological times. The resistance of the wood to 

 the organisms of decay is greater than that of any other common 

 plant tissue except those possessing cutinized or suberized cell walls. 

 We have, consequently, in the woody structures past and present 

 an almost perfect biological document, carrying back the history 

 of plants in relation to their changing conditions of environment 

 into remote epochs of our earth's history. 



In beginning the discussion of the organization of wood it will be 

 well to direct our attention in the first instance to the contrasts in 

 structure presented by woods of ancient and modern types. The 

 first illustration (Fig. 10) shows us the situation in the oak. The 

 wood is conspicuously marked into areas -by boundaries running 

 at right angles. Crossing the figure from top to bottom are rows 

 of large openings which represent the vessels or water-conducting 

 tubes of our modern forest trees. It is clear that these are large 

 only along the lines which mark the beginning of each year's growth. 

 Farther out the vessels become suddenly of much smaller caliber. 

 Not only is the ligneous structure of the oak transversely banded 

 by reason of the strikingly larger size of the vessels which signalize 

 the spring development, but also by an equally significant change 

 in the diameter and the distribution of other more or less highly 



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