192 THE CARBOHYDRATES 



the calcareous incrustation of certain Algae, e.g. Neomeris 

 dumetosa, is dependent on the presence of mucilage ; mucilage 

 provides a water-storage mechanism in plants subjected to 

 xerophytic conditions, e.g. Cassia obovata, Malva parviflora, 

 Theobroma cacao, and Pterocarpiis saxatilis ; finally, it may be 

 an important aid in connection with seed-dispersal and ger- 

 mination, as in some species of Salvia and Lepidium. 



Related to the gums and mucilages are the substances 

 known as galactans occurring in the seeds of Leguminosae 

 {Lupinus, Medicago, etc.) ; wood gum or xylan, occurring in 

 wood, etc. These substances have already been dealt with. 



PECTIC BODIES. 



The term pectin was first applied by Braconnot * to the 

 mucilaginous substance which he precipitated by means of 

 alcohol from the juices of many fruits and from aqueous 

 extracts of fleshy roots such as beet, carrot, swede, etc. 

 Similar substances were later found to obtain in a great variety 

 of plants such as onion, pea pods, leaves and stalks of cabbage, 

 rhubarb, and flax, and also in young cells such as the root 

 hairs of cabbage, cucumber, bean, and other plants. f In all 

 these cases the pectin occurs in a state of solution in the cell 

 sap or in association with the cellulose of the cell walls of 

 parenchymatous tissues. 



The name pectin was chosen because it was recognized that 

 these substances were in some way connected with the jellying 

 properties of fruit juices, Tre/cru? being the Greek for jelly. 



Fremy J was the first to show that unripe fruits contain 

 an insoluble precursor of the soluble pectin to which he gave 

 the name pectose ; as the fruit ripens the insoluble pectose 

 is gradually converted into soluble pectin, a change which is 

 revealed under the microscope by the swelling of the thickened 

 walls which become translucent and exude a mucilaginous 

 pectin. 



Somewhat similar changes are brought about by boiling 



* Braconnot, " Ann. d. Chim. et Phj^s.," 1824, [2], 28, 173. 

 t Howe : " Bot. Gaz.," 1921, 72, 313. 



I Fremy : " J. de Pharm.," 1840, [2]. 26, 368 ; and " Ann. d. chim. 

 et d. Phys.." 1848. [3]. 24, 5. 



