100 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



spiral and annular vessels, (7) the small pitted 

 tracheids (differing from vessels in not having their 

 end- walls absorbed), and (8) the xylem parenchyma in 



Fig. 30. Part of a Tangential Longitudinal Section of Stem of Maize, showing 

 one of the Bundles. A and E, parenchyma (ground tissue) ; B and D, 

 sclerenchyma ; C, xylem note the large pitted vessel on either side, and the 

 small pitted vessels' in the middle. 



the xylem; (9) the narrow sieve-tubes and (10) nar- 

 rower companion-cells in the phloem. 



1O9. Further Work on Herbaceous Stems. In examining 

 sections of various other herbaceous stems, or the youngest parts of 

 woody stems, note any special points of structure presented. 



In some herbaceous Dicotyledons, there is little or no inter- 

 fascicular cambium ; in Buttercup, etc. , each bundle is surrounded 

 by a more or less complete sheath of sclerenchyma, and even the 

 fascicular cambium is scanty and soon ceases to be active. 



The distribution of the stereome (strengthening or supporting 

 tissue) of herbaceous stems is of great interest. This term is often 

 restricted to the sclerenchyma, consisting of fibrous cells which 

 have thick lignified walls and have lost their living contents, so 

 that they serve a purely mechanical function. Collenchyma, 

 however, is an important form of supporting tissue, found below 

 the epidermis in herbaceous stems, young woody stems, petioles, 

 and flower-stalks ; its cells are living, usually with chloroplasts as 

 well as protoplasm, and the walls are thickened but not lignified, 

 hence this tissue can also perform vital functions, and it has the 

 power of growing, especially when subjected to tension. Note that 

 collenchyma is often developed chiefly in the projections of angular, 

 ribbed, or winged stems. 



The hypodermal sclerenchyma iisually forms a number of 

 isolated strands e.g. various Umbellifers, Leguminosae, Sedges, 

 Rushes. Pericyclic sclerenchyma may either form (1) a con- 



