1 8 THE ANATOMY OF WOODY PLANTS 



the pitted element, marking a distinct advance on the scalariform 

 and reticulate tracheids of the primary ligneous organization of 

 the ferns and their allies. In the case of the primary wood those 

 elements which have in their walls thickenings of the nature of 

 rings and spirals are ordinarily designated the protoxylem. This 



FIG. 13. Diagrammatic longitudinal section 

 of the fibrovascular tissues of a dicotyledon (after 

 Sachs), showing organization of primary wood. 



characterization of the first-formed portion of 

 the primary wood is not always justified by the 

 structures present, because in slowly growing 

 organs and in subterranean parts even of rapid 

 development typical ringed and spiral elements 

 may be nearly or quite absent. Usually, how- 

 ever, the protoxylem as defined is the ligneous 

 structure present when the organ is undergoing 

 rapid elongation and by its constitution permits 

 of accommodation by stretching to correspond 

 with the increase in length. Its elements as a 

 consequence are frequently drawn out and almost 

 obliterated. The cells of the primary wood 

 which are thickened in the reticulate, scalari- 

 form, or pitted manner are formed after elongation has ceased, 

 since by their organization they are incapable of increasing their 



A 



FIG. 14. Dia- 

 grammatic views 

 of a young ele- 

 ment of the pri- 

 mary wood. In A 

 the normal condi- 

 tion is shown, 

 while in B the pro- 

 toplasm has been 

 caused to contract 

 by means of plas- 

 molysis. 



