112 PHYSIOLOGY OF NUTRITION. 



Potassium chlorate in a wide test tube, and add enough Xitric 

 acid to cover them completely. We then drop into the mixture 

 not too thin longitudinal sections from the wood of a twig of 

 Tilia, 5 mm. thick, and heat over a spirit name till a brisk evo- 

 lution of gas takes place. After allowing the reaction to proceed 

 for a few minutes longer, we pour the entire contents of the test 

 tube into a large quantity of water, remove the floating sections 

 with a glass rod, wash them with clean water, and lay them in a 

 drop of water on the slide. By the maceration method the middle 

 lamella between neighbouring elements is destroyed, and conse- 

 quently the preparations may now be easily teased out with 

 needles, so that the isolated elements of the wood can be separately 

 observed. We recognise particularly the presence of many wood 

 fibres and vessels (these latter have partly fallen to pieces), but 

 even thin-walled wood parenchyma cells and tracheides are not 

 altogether absent (see Fig. 34). 



1 See de Bary, Comparative Anatomy of the Vegetative Organs of the 

 Phanerogams atid Ferns, p. 74. 



2 For further reactions, see v. Holm el in Sitzungsber. d. Akadem. d. Wiss. 

 zu Wien., Bd. 76, Erste Abtheiluug, p. 507. 



3 Wiesner, Si.lzber. d. Akadem. d. Wiss. zu Wien., Bd. 77, 1. Abth., 1878, p. 

 60. For details respecting lignified and suberised membranes, see Zimmer- 

 mann, Die botani*che Mikrotechnik, 1892, p. 140, where also the literature is 

 given. 



4 On the chemical character of the substances causing lignificatiou of tbe 

 membranes, see Singer, Sitzungsber. d. Akadem. d. Wiss. zu Wien, 1882, Bd. 85, 

 p. 345. An important constituent of the lignified membranes of plant cells, 

 whose presence may perhaps cause the reactions with phloroglucin and aniline 

 sulphate, is vanillin. According to my view, however, the recent work of 

 Lange, Thomsen, Hegler and others, has not yet settled the question of the 

 chemical nature of wood substance. 



43. Starch Grains. 



A small quantity of air-dry potato meal is placed in a drop of 

 water on a slide, and covered with a cover-glass. Or we halve a 

 potato, scrape the cut surface with a knife, and examine the 

 scrapings under the microscope. The starch grains of the potato 

 are of very different sizes, some of them becoming relatively very 

 large. They are excentric, i.e. the organic centre round which 

 the layers are arranged does not correspond with the geometrical 

 centre (see Fig. 35). 



