29.] 



CHAPTER II. THE TISSUES. 



135 



complete or partial absorption of the intervening walls (Fig. 95 



Co) ; 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 Phane- 



rogams. 



Tracheal tissue is the character- 

 istic constituent of the vascular 

 tissue-system to be described sub- 

 sequently ; it is especially adapted 

 for the conduction of water. It 

 should be noted that in all forms 

 of lignified tissues, whether tra- 

 chea!, sclerenchymatous, or paren- 

 chymatous, the middle lamella is 

 the most highly lignified part (p. 

 128) ; it dissolves readily in a 

 mixture of nitric acid and chlorate 

 of potash, thus leading to the iso- 

 lation of the constituent cells. 



When a tracheal cell with a 

 pitted wall abuts upon cells con- 

 taining living protoplasm, it not 

 unfrequently happens that the thin 

 pit-membranes begin to bulge, in consequence of the pressure upon 

 them of the contents of the living cells, into the cavity of the 

 tracheal cell, and actually 

 grow (Fig. 96). Cell-di- 

 vision may take place in 

 these ingrowths, so that a 

 mass of cellular tissue is 

 formed in the cavity of 

 the tracheal cell. These 

 ingrowths are termed 

 tyloses ; they are constant- 

 ly to be found in some 

 kinds of wood, (e.g. Bo- 



FIG. 95. Tracheal tissue. A Tracheid 

 from the leaf of Sphagnum. JThe holes 

 in the external wall. B Tracheid from 

 the leaf of Polypodium vulgar -e. 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 arti- 

 culations, where the absorbed transverse 

 walls existed. 



cm 



binia) and occasionally in 

 many others. 



4. Sieve-Tissue. This 

 tissue consists mainly of long articulated tubes, the contents of 



Fm. 96 (after Weiss) .Portion of wood-vessel of 

 Vitis vinifera, in transverse section, with adjacent 

 cells a b c, one of which has grown into the lumon 

 of the vessel forming a tylose b. 



