35^ 



PRIMARY ARRANGEMENT OF TISSUES. 



part of the row by one or more layers of interstitial cells. In the thicker polyarch 

 bundles these large vessels are often confined to certain rows, while in the others, 

 which alternate irregularly with the former, they are absent ; or the case frequently 



Fig. i66.— Acorus Calamus. Cross-section through a vascular bundle and the neighbouring part of the cortex of 

 an adventitious root, s endodennis ; /, / narrow primitive vessels ; ^ larger internal vessels, not yet completely 

 developed ; /A phloem-groups. From Sachs* Textbook. 



r^ 



Fig. 167. — Very thin crobs-section through the vascular bundle of an older adventitious root of the same plant 

 (145). s endodermis, g primitive vessels, -w phloem groups. The axial mass of cells, which is still thin-walled in 

 Fig. 166, is here sclerotic, and the internal vessels are completely developed. 



occurs that two neighbouring rows converge at an acute angle towards a large vessel, 

 forming in cross-section the figure of a V, in the angle of which the large vessel lies. 

 Xylem-plates do, however, occur among Monocotyledons also, in which the elements 



