162 



XI. THE DICOTYLEDONOUS STEM WOODY TYPE. 



the Bastard Acacia (Robinia Pse^ld- Acacia), in which they occur 

 with remarkable regularity and frequency. Selecting a piece of 

 old branch, say ten or twelve years old, w T e remove from it peri- 

 pheral strips, penetrating into the wood to about the depth of the 

 fourth year, and can discard the remaining central portion. 

 These strips can be preserved in alcohol, but must then be 

 treated, before working, with glycerine-alcohol to soften them. 

 Cross-sections can be taken passing from the cambium inwards, 

 and examined first with a low power. We shall soon note' that 



the broader vessels, from about 

 the third year's ring inwards, 

 are filled with a thin-walled 

 tissue (Fig. 64). We can 

 generally find vessels in which 

 the origin of this tissue can be 

 followed. W T e can see bladder- 

 like structures arise from one 

 or several places on the wall 

 of the trachea, and protrude 

 into the cavity (at a). It is 

 these bladders which, enlarg- 

 ing and pressing upon one 

 another, and flattening on 



their contact sides, fill the 

 FIG. 64. A vessel filled with a Tracheal , . 



plug, together with the adjoining cells, from whole vessel With tissue. At 

 the duramen of Robinia Pseud-Acacia, in < m p/inll v fnvrmrnHlo !Q^AQ ir. 

 cross-section. At a and a, the connection S P 6< m > HlVOUiable places in 

 of the plug cells with their mother cells can the cross-section we can deter- 

 beseen(x 300). . ...... 



mine that it is the adjoining 



wood-parenchyma cells which protrude, bladder-wise, through 

 individual bordered pits into the cavity of the vessel. It is the 

 unlignified closing membrane of a unilaterally- bordered pit which 

 is concerned in the process. These plugs are found in the stems 

 of a few Monocotyledons, many Dicotyledons, and occasionally 

 in Conifers. In woody roots they are rare, but common in her- 

 baceous roots, as, e.g., Cucurbita. 



