CONTINUITY OF SECONDARY HADROME 673 



of cells, namely, ordinary medullary ray cells (conducting parenchyma) 

 and water-conducting tracheidal cells. That the latter should com- 

 municate by means of bordered pits with the longitudinal fibrous 

 tracheides, is quite in accordance with the general principle of con- 

 tinuity underlying the construction of the conducting system. The 

 typical medullary ray cells are, however, likewise connected with the 

 adjoining fibrous tracheides by pits, which are often of considerable 

 size. These pits are particularly large in the genus Pinus, where each 

 extends almost across the whole width of the medullary ray cell (Fig. 

 276, ml). The presence of such large pits greatly reduces the strength 

 of the tracheidal walls ; the tracheides accordingly develop remarkable 

 transverse buttresses, which preserve them from compression at the 

 hands of the turgescent medullary ray tissue. Each of these buttresses 

 takes the form of a short rod bearing a disc-like expansion at either 

 end ; in longitudinal section, therefore, the buttress is shaped like an 

 H, the upright arms, which correspond to the terminal discs, being 

 attached to the lateral walls of the tracheide. 



Among Dicotyledons also, the cells of the medullary rays com- 

 municate with the adjacent vessels by means of pits ; this statement 

 applies more particularly to the upright marginal cells, where these are 

 specially differentiated. In the genera Salix and Populus, the pits are 

 elliptical and slightly bordered. Among the Cupuliferae, and in a 

 variety of other trees aud shrubs, the pits are fairly large in this 

 respect agreeing with those of the Salicaceae but show no trace of a 

 border. They are larger still (01012 mm. in diameter) in Morus 

 alba, Aristolochia Siplio, Vitis vinifera and Step// ylea pinnata; in surface 

 view these exceptionally large pits no longer present the usual circular 

 or elliptical outline, but are seen to be greatly elongated or irregular in 

 cross-section. Among the Posaceae especially in certain Pomaceae - 

 and Legtbiinosae, on the other hand, the corresponding pits are small, 

 simple and irregularly distributed. 



The question now arises as to the physiological significance of the 

 above-described connection between conducting parenchyma and vas- 

 cular tissue. The anatomical data certainly point to one of two 

 conclusions : either the xylem-parenchyma and the medullary rays 

 assist in the conduction of water, or else the vessels and tracheides 

 take part in the translocation of carbohydrates. The two views are 

 not mutually incompatible, and it is quite possible that they are both 

 correct. The possibility that the conducting parenchyma participates 

 in the transportation of water, has already been considered on a 

 previous occasion (Chap. VII.). It may be added at this stage, that the 

 production of exudation-pressure which may be set up in the water- 

 conducting tissues of the woody cylinder in axial organs as well as in 



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