MORPHOLOGY 



115 



tions of the vascular bundles are continued, the bundles themselves become 

 attenuated and simpler in structure (Fig. 126). The vessels first disappear, and 

 only spirally and reticulately thickened tracheides remain to provide for the water 

 conduction. The phloem elements undergo a similar reduction. In Angiosperms, 

 in which the sieve-tubes are accompanied by companion cells, the sieve-tubes become 

 narrower, whilst the companion cells retain their original dimensions. Finally, 



FIG. 125. Transverse section of a concentric bundle from the petiole of 1'teris aquilina. 'sc, 

 Scalariform vessels ; sp, protoxylem (spiral tracheides) ; sc*, part of a transverse wall showing 

 .scalariform perforations ; Ip, xylem parenchyma ; v, sieve-tubes ; j>r, protophloem ; pp, starch 

 layer; e, endodermis ; s, phloem parenchyma, (x 240.) 



in the cells forming the continuation of the sieve-tubes, the longitudinal division 

 into sieve-tubes and companion cells does not take place, and TRANSITION CELLS are 

 formed ( 112 ). With these the phloem terminates, although the vascular portion of 

 the bundles still continues to be represented by short spiral tracheides. The 

 ultimate branches of the bundles either terminate blindly or anastomose with other 

 vascular bundles. 



The Fundamental Tissue System usually forms the main bulk 

 of the primary tissues of the body of a plant. The whole tissue of 

 the lower plants, as it shows no internal differentiation, may, in a 

 certain sense, be considered fundamental tissue. The other tissues 



