912 A TEXTBOOK OF THEORETICAL BOTANY 



A concentration of smaller trace bundles and associated sclerenchyma 

 strands at the periphery of the stem forms a mechanically strong rind, which 

 in these stems takes the place of periderm. The inner tissues are relatively 

 unimportant mechanically. 



A small group of genera in the Agarales also produce woody stems, notably 

 Yucca, Aloe, Dracaena and Cordyline, by a different and peculiar process. 

 We have already mentioned the existence of a bundle cambium in many 

 Monocotyledons, which produces a very limited amount of secondary xylem, 

 though no continuous cambium is formed. In the genera cited above a broad 

 meristematic zone arises in the inner cortex, closely surrounding the central 

 group of primary bundles, that is to say, it occupies the position of a peri- 

 cycle (Fig. 894). In some species this zone is already differentiated in the 

 apical meristematic region, while in others it appears only towards the base 

 of the main stem, where all the tissues are fully differentiated. It is composed 

 of squarish, thin-walled parenchyma, arranged in radial rows (Fig. 895). It 

 resembles a cambium and is usually so called, but it differs both in origin, 

 structure and behaviour from typical cambia. The radial rows are not the 

 products of division of a single initial cell, but are built up by the successive 

 divisions of three, four or more cortical cells in radial files. The individual 

 cells are not cambiform and do not segregate xylem inwards and phloem 

 outwards, but produce entire, closed, fibro-vascular bundles, within the zone, 

 by the sub-division of single or of grouped meristematic cells. These new 

 bundles arise close together in centrifugal succession, and as they mature the 

 parenchyma between them becomes lignified, so that a remarkably solid 

 band of secondary tissue may be built up in the course of years. Old stems 

 of Aloe and some other genera have even been observed to show annual rings 

 of growth. 



The Mechanical Properties of Tissues 



The sclerenchymatous tissue of plants is regarded as mechanical in 

 function, adding strength to the various organs, and it has been classified as 

 a distinct tissue system, under the name of stereome. It is naturally best 

 developed in organs which have comparatively little wood, such as herbaceous 

 and monocotyledonous stems. Besides sclerenchyma the stereome includes 

 also the collenchyma, which is inferior in strength to the sclerenchyma but 

 is more flexible. Schwendener in 1874 published his researches into the 

 mechanical properties of stereome tissues and showed how their distribution 

 fulfilled certain mechanical principles of construction, suitable to the type 

 of strain to be encountered. In stems these are mostly transverse, bending 

 strains, and in roots longitudinal, pulling strains. 



For the end to be served the properties of plant fibres are suitable. Thus 

 they have a high modulus or limit of elasticity, and they can suffer consider- 

 able elongation without permanent deformation, but beyond this limit, they 

 soon reach their breaking point, further resistance being probably valueless 

 after permanent elongation has occurred, which must inflict irreparable 



