PART II. ANATOMY AND HISTOLOGY. 



[28 



and in the leaves and cortical tissue of the stem of Sphagnum 

 (gametophyte) where they are not lignified at all (Fig. 73 A). 



b. The tracheae, are true vessels, that is, syncytes (p. 64), the 

 contiguous cell-cavities having been rendered continuous by the 

 complete or partial absorption of the intervening walls (Fig. 73 

 C a) ; the former is more frequently the case when the interven- 

 ing walls are horizontal, the latter when they are oblique ; they 

 occur in the wood of many Phanerogams. 



Tracheal tissue is the characteristic constituent of the vascular 

 tissue-system to be described subsequently ; it is especially adapted 

 for the conduction of water. It should be noted that in all forms 



of lignified tissues, whether 

 tracheal, sclerenchymatous, or 

 parenchymatous, the middle 

 lamella is the most highly lig- 

 mfied part (p. 88) ; it dissolves 

 readily in a mixture of nitric 

 acid and chlorate of potash, thus 

 leading to the isolation of the 

 constituent cells. 



When a vessel with a pitted 

 wall abuts upon cells containing 

 living protoplasm, it not un- 

 frequently happens that the 

 thin pit-membranes begin to 

 bulge, in consequence of the 

 pressure upon them of the con^" 

 tents of the living cells, into the 

 cavity of the tracheal cell, and 

 actually grow. Cell - division 

 may take place in these in- 

 growths, so that a mass of cel- 

 lular tissue is formed in the 

 cavity of the vessel. These in- 

 growths are termed tylosesj they are constantly to be found in 

 some kinds of wood (e.gr.'Robinia), and occasionally in many others. 

 6. Sieve-Tissue.. This tissue consists mainly of long articulated 

 tubes, the contents of the contiguous segments communicating by 

 means of the sieve-plates, which usually lie on the transverse 

 walls, either singly, when the transverse wall is horizontal, as 

 generally in herbaceous plants (Fig. 74), or several together, when 



FIG. 73. Tracheal tissue. A Tracheid 

 from the leaf of Sphagnum ; j The holes in 

 the external wall. B Scalariform tracheid 

 from the leaf of Polypodium vulgare. C 

 Part of a trachea with bordered pits from 

 the stem of Helianthus; it has been cut 

 into at the upper end ; a and b the articula- 

 tions, where the absorbed transverse walls 

 existed. 



