64 V. CELL-WALL, AND CELL CONTENTS. 



Reactions of Cellulose. The structural relations in the cells of 

 the beetroot show up still more beautifully and distinctly if the 

 section is treated for some time in a watch-glass with a solution 

 of logwood diluted with water. All the cell-walls are then 

 coloured violet, excepting the strongly lignified walls of the vessels. 

 On the parenchyma-cells the pit-surfaces are either not stained 

 or else show a slightly stained network. Unlignified membranes 

 which are stained with logwood owe this coloration to the pectin 

 with which the cellulose is mingled. The nucleus also stains 

 pretty deeply. The vessels contain neither nuclei nor plasmic 

 contents. If a section is laid in chlorzinc iodine the characteristic 

 violet cellulose-reaction soon appears. The colour begins at the 

 edges of the section, and progresses slowly inwards, but is often 

 not complete for hours ; the lignified thickening layers of the 

 vessels stain yellow-brown. If we are dealing with alcohol mate- 

 rial soaking in water will hasten the reaction. On the walls of 

 the cells, the surfaces of the pits once more remain unstained, 



sulphuric acid (best, dilute 50 per cent.), the resulting sulphate of lime, instead 

 of being dissolved, may crystallise out in the form, usually, of fine needles. 

 These will probably be found spread through or over many cells, surrounding 

 the original crystallogenous cell ; but if it be desired to change oxalate into 

 sulphate of lime in situ it can be done by heating the section up to boiling- 

 point in dilute sulphuric acid (2 acid, 1 water). The resulting masses of 

 gypsum crystals often occupy the same space and form as the original calcium 

 oxalate crystal. This is particularly striking where the crystals have a well- 

 marked form, as, e.g., the quadrate octohedra in the stem and petiole of many 

 species of Begonia. 



Hydrochloric acid has also been much used as a solvent of oxalate of lime 

 without evolution of gas, but the large crystalline masses in which the salt is 

 usually found are as a rule embedded in a mucilaginous jelly, which retards 

 the action of the acid, though it can be hastened by warming. 



Calcium oxalate crystals are very doubly refractive, and hence show up 

 beautifully under the polariscope, glistening brilliantly upon the dark back- 

 ground when the prisms are crossed by rotation. 



Calcium carbonate, occasionally met with in the form of a deposit in the 

 cell, as, e.g., the large cystoliths in special epidermal cells of the india-rubber 

 plant (Ficus elastica), but more commonly as a deposit in the interior of 

 cell-walls, or as an external crust to the plant (many water plants and many 

 Saxifrages, e.g., outside the water-pores of S. cr-ustata) is best tested by its ready 

 solubility in dilute, not concentrated, acetic acid, with evolution of bubbles 

 of gas, carbonic acid gas ; or concentrated, not dilute, hydrochloric acid may 

 be used with similar results. Sulphuric acid forms gypsum crystals, as with 

 the oxalate. 



All these reactions are best performed with micro-preparations. [ED.] 



