78 BOTANY part i 



Van Wisselingh has shown that phellonic acid, which is always present in 

 suberin, is constantly absent in cutin. The behaviour of cutin, as of suberin, varies 

 according to the source from which it is derived. Cutin withstands better the 

 action of caustic potash. In other respects, the reactions given by cutiuised cell 

 walls with chlor-zinc-iodide or solutions of caustic potash are almost identical 

 with those of suberised cell walls. Both are insoluble in sulphuric acid and in 

 cuprammonia. 



Young cell walls are less elastic, but relatively more extensible 

 than older ones. The power of resisting a stress is increased by 

 lignification. The presence of cutinised and corky membranes 

 diminishes the loss of water from the surface of the plant. 



The layers of the cell walls of some cells, particularly the super- 

 ficial cells of certain fruits, as of Sage, and of numerous seeds, such as 

 Flax and Quince seeds, become mucilaginous, and swell in water to 

 MUCILAGE, which serves the purpose of attaching the seeds to the 

 soil. Firm cell walls can also be transformed into GUM, as is so 

 often apparent in Cherry and Acacia trees, portions of the wood of 

 \vhich often succumb to gummosis (p. 69). 



In such cases, however, only a small amount of the gum is derived by the 

 change of the cell walls. In the case of Cherry gum this part is insoluble in water, 

 while the gum formed within the cells is soluble (®^). 



The cell walls of the seeds of many Palms, as also those of Ornithogaluvi 

 (Fig. 77), have strongly developed thickening layers, which are full of pits. These 

 thickening layers are lustrous white, and, as in the case of the seeds of the Palm, 

 Phijtclephas macrocaiya, may attain such a degree of hardness as to be technically 

 valuable as vegetable ivory. Such thickening layers may contain other carbo- 

 hydrates in addition to cellulose, and by the action of ferments are dissolved 

 during germination. They are accordingly to be considered as a reserve substance 

 of the seeds. 



Cell walls often become coloured by derivative substances of 

 tannin ; in this way, for instance, the dark colour is produced which 

 is often seen in the coats of seeds and in old wood. The colours of 

 the woods of economic value are due to such discoloured cell Avails. 

 Inorganic substances are often deposited in large cpiantities in old cell 

 walls. Among such substances calcium oxalate is often met with, 

 commonly in crystal form ; also, though not so frequently, calcium 

 carbonate. In the cystoliths of Ficus elastica (Fig. 83) so much 

 calcium carbonate is deposited that it effervesces with hydrochloric 

 acid. In some plants, as, for instance, most of the Characeae, the 

 quantity of calcium carbonate in the cell walls is so great as to 

 render them stiff and brittle. Silica is also present in the superficial 

 cell walls of the Gramineae, Equisetaceae, and many other plants, and 

 gives them a very considerable firmness. The lens-shaped thickenings 

 in the outer walls of the epidermal cells of Campanula persicifulia are 

 also silicified (Fig. 82). 



By withdrawing water from the cells a contraction of the protoplast and its 



