58 THE ANATOMY OF WOODY PLANTS 



a tylosis. An adhesion of the pit membranes to one overhanging 

 margin or the other of the pit cannot be made out. Occasionally 

 a torus is present in the relatively narrow membranes of the small 

 bordered pits of the water-conducting elements of the dicotyledons, 

 but it does not present the phenomenon of fusion with the margins 

 of the pit characteristic of heartwood in conifers. The paren- 

 chymatous constituents of dicotyledonous woods in many cases give 

 rise to highly efficient antiseptics in the case of the heartwood. 

 In many instances, such as the oak, the blue gum, the quebracho, 

 etc., large amounts of tannin are formed which serve as an effectual 

 preservative. In other cases ulmic and even humic acids make 

 their appearance and exercise greater or less inhibitive action on 

 the organisms which ordinarily bring about the decay of woody 

 structures. In teak we have the rare example of a structurally 

 valuable dicotyledonous wood which in the transformation from 

 heartwood to sapwood elaborates, not acid substances which exer- 

 cise a corrosive action on metals, particularly on iron and steel, 

 but an essential oil. The enduring heartwood of the teak (Tectona 

 grandis) is consequently valuable above all others for naval con- 

 struction by reason of its compatibility with iron and steel, since it 

 neither corrodes this fundamental structural material of present 

 naval architecture nor, in turn, is rotted by iron rust. 



It will be apparent from the foregoing paragraphs that those 

 longitudinal elements which subserve the function of storage in 

 the woods of Mesozoic, Tertiary, and actual plants are of great 

 evolutionary significance. Their importance in this respect 

 can be gauged only after the rays or radial storage devices have 

 been considered in the next chapter, and they will receive their 

 final and fullest appreciation in connection with the highest groups 

 of plants. It is obvious that the incentive to the development of 

 longitudinal parenchymatous elements was the appearance of an 

 annual winter period of rest which in later geological times, begin- 

 ning with the earlier Mesozoic, with progressively greater emphasis 

 marked the originally unvarying cycle of the year. The first 

 parenchymatous elements came into being, so far as our knowledge 

 at present goes, in the earlier Jurassic. Their primitive occurrence 

 was at the end of the annual ring. In the conifers in this position 



