SKCT. i MORPHOLOGY 71 



or related substances, such as the mucilage of the seeds of the Cruciferae and Quince 

 (Cydonia), that from the mucilage cells of the Malveae, the gums of the Cherry 

 and Acacia, the gum tragacanth from Astragalus gummifer. The mucilage of 

 Orchid tubers, on the other hand, is related to cellulose, and remains uncoloured 

 with the same reagent. 



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

 (Fig. 71), 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, 

 I'/iiff: Icphas macrocarpa, 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 tannin or derivative sub- 

 stances ; 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 walls. 

 Inorganic substances are often deposited in large quantities in old cell 

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

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

 carbonate. In the cystoliths of Ficus elastica (Fig. 76) so much 

 calcium carbonate is deposited that it effervesces with hydrochloric 

 acid. In many 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 persicifolia are 

 also silicified. 



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

 consequent separation from the cell wall is brought about (see Plasmolysis). Such 

 protoplasts are able under certain conditions to surround themselves with a new 

 cell -membrane. The removal of the cuticle or of the waxy covering from the 

 surface of certain plants (Agave, Aloe, Ricinus, Sedum) is followed by its re 

 generation ( 60 ). 



Form of the Cell. As cytoplasm is a viscous fluid, and would tend, 

 if unimpeded, to take a spherical shape, it may be assumed that the 

 natural and primary form for cells is spherical. Such a shape, 

 however, could only be realised by cells which, in their living 

 condition, were completely free and unconfined, or in such as were 

 able to expand freely in all directions. Newly developed cells, 

 in a continuous tissue, are, at first, nearly always polygonal. 

 Through subsequent growth their shape may change. The cubical 

 cells of the growing point either elongate to a prism or remain, 

 owing to repeated division, short and tabular. If the growth is 

 limited to certain regularly arranged points of the surface, they 



