FIBRO VASCULAR TISSUES: TRACHEIDS AND FIBERS 35 



of the elements in question. The condition here present is repre- 

 sented in Fig. 27. To the left (a) is shown a septate fiber 

 from the grapevine, while on the right appears a similar element 

 from teak (Tectona grandis). Elements of this nature are the 

 probable basis of Strasburger's apparently badly founded view that 

 parenchyma cells by their fusion have given rise to libriform fibers. 

 The pits in septate fibers are usually much larger than in the libri- 

 form elements and are conspicuously simple. 



A last interesting category of fibrous elements, also characteristic 

 of extreme types, such as herbs, shrubs, and vines, is the so-called 

 substitute fiber (Ersatzfaser of German authors). In elements of 

 this nature septation is absent, but protoplasmic contents are 

 present in the cell when it is fully developed and in a functional 

 condition. Fig. 28 shows such an element from the wood of the 

 barberry. The pits are both radial and tangential, although the 

 former are obscured in illustration by the presence of protoplasm. 

 The mechanical elements of nearly all dicotyledonous herbs belong 

 in this category. The substitute fiber represents a convenient 

 union of the functions of strength and storage of food in the same 

 element, a condition particularly advantageous in the slender stems 

 of vines and herbs. 



The final paragraph of this chapter may appropriately be 

 devoted to the subject of the distribution of the pitting in the fibers 

 of the dicotyledons. It has been noted that in the conifers in 

 general the pitting of the tracheary or fibrous elements is typically 

 confined to the radial walls, except in the case of the cells termi- 

 nating the year's growth (summer tracheids). The mechanical 

 elements of the end of the annual ring have tangential as well as 

 radial pits. In the case of the Paleozoic gymnosperms, since 

 there are (with certain unimportant exceptions) no annual rings 

 marking seasonal diversity, tangential pitting is entirely absent, and 

 the pores of the tracheids are, as a consequence, exclusively radial. 

 The dicotyledons and the Gnetales present a very different situation 

 from that found in the seed plants characteristic of Mesozoic and 

 Paleozoic times; for in the former the mechanical elements, 

 whether of the nature of tracheids, fiber-tracheids, libriform fibers, 



