1918] Hodgson: Foliar Abscission in Citrus 425 



substances. These have been divided into simple and compound 

 celluloses. The more resistant to hydrolysis are known as the true 

 celluloses, the less resistant simple celluloses being termed hemicellu- 

 loses. The compound celluloses have been further divided and we 

 now have the pectose-celluloses or pectoses which are believed to con- 

 stitute the middle lamella (cf. Atkins, 1916). Evidence tending to 

 show that the middle lamella itself consists of calcium pectate and the 

 inner lamellae of pectose has been advanced (Bertrand and Mallevre, 

 189-4, 1895 ; cf. Jones, 1909) . In the case of Citrus, there is no evidence 

 that separation is brought about by the dissolution of the middle 

 lamella alone but rather by the hydrolysis of the entire wall with the 

 exception of the delicate tertiary membranes (cf. Lloyd, 1916). It is 

 probable that we have to do here with a mixture of calcium pectate and 

 pectose. 



A number of enzymes acting on pectic compounds have been 

 isolated. In 1840 Fremy isolated an enzyme "pectase" from carrot 

 roots which was capable of gelatinizing pectose and related compounds 

 (Fremy, 1840). He supposed that the reaction involved was the con- 

 version of the pectose into pectic acid. However, more recent 

 researches have shown that the coagulum consists of calcium pectate 

 and that in reality the reaction represents the coagulation of pectin in 

 the presence of calcium salts (Bertrand and Mallevre, 1894, 1895). 

 Brown and Morris (1890) obtained from barley malt an enzyme of a 

 cytolytic nature, in addition to diastase. It was found to function in 

 the dissolution of the endosperm cell walls preceding the action of 

 diastase upon the starch. The description of the process as given by 

 them is very similar indeed to that which occurs in the separating cells 

 in Citrus and in Mirahilis (cf. Lloyd, 1916). A number of other inves- 

 tigators have found evidence of the presence of enzymes of this char- 

 acter in germinating seeds. It remained for Bourquelot and Herissey 

 (1898) to try the action of barley malt extract upon a solution of 

 pectin. They obtained evidence of the presence of an enzyme which 

 so changed the pectin that it could not thereafter be coagulated by the 

 action of the enzyme pectase. This new enzyme they called pectinase. 

 It was later shown that when the coagulum resulting from the action 

 of pectase on pectin was treated witli pectinase it was dissolved and 

 coincidently traces of reducing sugars were found, showing that tlie 

 process is a hydrolytic one '(Bourquelot, 1899). Jones (1909) has 

 isolated a cytolytic enzyme from Bacillus carotovorus .\.\\'n-\\ lie calls 

 pectinase. According to him tliis enzyme attacks the middle laiuellae. 



