524 PHYSIOLOGY OF GROWTH. 



they play the part of mechanical tissue. In the haulm of 

 Molinia coerulea a closed ring of mechanical tissue is present, 

 which is further strengthened by subepidermal ribs. The vas- 

 cular bundles are partly imbedded in this ring and partly sur- 

 rounded by it. The haulm of the rye is similar in construction. 



It is also of interest to make ourselves acquainted with the 

 mechanical tissue of the leaf sheaths of grasses. These tubular 

 sheaths embrace the stem part of the haulm, and their special 

 function is to shelter the delicate growing regions of the stem, 

 which, as is well known, are situated in grasses at the base of the 

 internode. We prepare delicate transverse sections from the part 

 of a rye haulm lying immediately above a young node. Below 

 the epidermis of the inner side of the leaf sheath we perceive a 

 tissue free from chlorophyll, but under the epidermis of the 

 outer side, chlorophyll-containing tissue. The vascular bundles 

 are readily recognised ; they are provided both inside and outside 

 with a well-developed layer of bast fibres. 



If we wish to acquaint ourselves with the arrangement of the 

 mechanical tissue in structures not resistant on all sides to flexion, 

 we prepare, e.g., transverse sections of the leaf of Phormium tenax 

 and through the mid-rib of a fully developed leaf of Zea Mais. 

 The structure of the Phormium leaf is not quite the same above 

 and below, but at all events the stereome bundles covering the 

 vascular bundles on the under and lower sides of the leaf at once 

 strike the eye. In Zea subepidermal masses of sclerenchyma are 

 present on the upper side of the leaf ; the mechanical tissue of the 

 under side of the leaf stands in exact relation to the grouping of 

 the large vascular bundles. 



As we have already remarked, rhizomes and roots, more especi- 

 ally, are constructed to resist tension and pressure. We prepare 

 a transverse section through the rhizome of Carex glauca. The 

 peripheral ring of sclerenchyma, not, it is true, very well de- 

 veloped, serves as protection against radial pressure. The central 

 cylinder of wood, consisting of thick-walled elements, and in 

 which most of the vascular bundles are scattered (a few bundles 

 also lie outside this cylinder), serves to secure resistance to 

 tension. We find aii entirely similar arrangement of the me- 

 chanical tissue on investigation of the transverse section of lateral 

 roots of the first order of Zea Mais. The peripheral ring of 

 sclerenchyma is here, however, developed much more highly than 

 in the rhizome of Carex. 1 



