THE SKELETON OF THE PLANT 423 



to the peculiar properties of the cuticle. Suberin is the sub- 

 stance found in great proportion in the walls of corky cells. 

 Both of these are formed like lignin by modification of cellulose. 

 They serve rather to prevent undue evaporation than to act as 

 a true skeleton. Li old trees, however, the complex barks, into 

 the composition of which cork enters to a very considerable 

 extent, often subserve somewhat the purpose of aiding in the 

 mechanical support of the tree, especially in cases wdiere the 

 woody centre decays and the tree becomes hollow. 



Li many cases the cell-walls, while remaining chemically 

 unchanged, become impregnated with silica or other mineral 

 matter. The cereal grasses and the Equisetaceae accumulate 

 copious quantities of silica in the walls of their epidermal cells. 

 Though this appears not to be a,ltogether essential to their power 

 of remaining upright, it no doubt materially assists it. 



Other moditications of cellulose are found in various cell- 

 walls, but they are comparatively unimportant from the pomt of 

 view now under discussion. 



Where the ordinary unmodilied cell-walls of the j^lant are 

 not sufficient to secure rigidity, as in most terrestrial plants, there 

 is a very varied distribution of tissues for that purpose. The 

 forms wdiicli are found are collenchjma, sclerenchjuna, and 

 vascular tissue, the former two appearing to subserve mechanical 

 purposes only, while the latter also takes part in the transport of 

 water. In delicate stems but little development of either is seen ; 

 thus most moss plants show in their axis only the hypodermal 

 cells thickened, while this is supplemented in the Pohjtricliacece 

 by a central core of somewhat lignified cells. In succulent 

 petioles there is a hypodermal development of collenchyma, in 

 addition to the meristeles, which contain a certain amount of 

 wood. In many herbaceous stems, particularly those with 

 angles, strands of sclerenchyma are found underlying part or 

 the whole of the margins. In many Monocotyledons of humble 

 growth the centre of the stem is hollow or is occupied by thin- 

 walled fundamental tissue, sometimes made of stellate cells, and 

 the margin is strengthened by strands of sclerenchymatous 

 fibres placed behind or between the libro-vascular bundles. 



Plants of larger growth depend very largely on the fibro- 

 vascular part of the stele for their supporting tissue. This is 

 seen most easily in the trees belonging to the Gymnosperms and 

 Dicotyledons, which increase regularly in thickness. Large 

 monocotyledonous trees depend upon the same class of elements, 

 though the distribution of the individual bundles is different. 



