426 University of California Piihlications in Botany [Vol.6 



causing first a gelatinization followed by dissolution, resulting in sepa- 

 ration of the cells in the tissue attacked. Atkins (1916) has recently 

 reviewed the literature on the action of enzymes on peetic substances. 

 He distinguishes three distinct enzymes causing as many different 

 reactions. According to him, pectose is split up by pectosinase to 

 form pectin which may be transformed into peetic acid by pectase. 

 The peetic acid may in turn be converted into sugars by pectinase. 



"Whereas the literature on the subject is somewhat confusing and 

 emphasizes our lack of definite knowledge regarding the composition 

 of the inner and middle lamellae, there is little doubt but that enzymes 

 capable of hydrolysing its various component parts have been isolated. 

 In the case of Citrus the swelling is so pronounced as to indicate that 

 something of the nature of gelatinization of the wall occurs, very 

 similar to the reaction of pectase on peetic compounds. The subse- 

 quent dissolution of this wall resembles markedly the action of pec- 

 tinase on peetic coagulums. 



There is little or no evidence that turgor plays any prominent part 

 in the abscission process, as Goodspeed and Kendall (1916) suggested 

 might be the case in Nicotiana (cf. Kendall, 1917). Observation of 

 the abscission process in very young leaves indicates how turgor might 

 be taken as the causal agent. In such tissues the cell walls are quite 

 thin and the swelling not at all conspicuous. Moreover, during sepa- 

 ration the cells round up beautifully indicating considerable release 

 of pressure. 



The process apparently occurs simultaneously in the cortex and 

 in the pith, all cells across the petiole in the region of the separation 

 layer taking part except the tracheae. In young stems the epidermal 

 cells and the wood parenchyma are seen to function very distinctly 

 in the process. The only mechanical break occurs in the wood vessels 

 and in the cuticle. No extenuation of the protoplasmic utricle of the 

 separation cells during abscission has been observed (cf. Lloyd, 1916), 

 and no evidences of cell divisions during the process have been found 

 although immediately following separation new cross walls are fre- 

 quently laid down in the cells of the separation zone. If moisture 

 conditions are optimum following separation, all of the cells in the 

 abscission zone may start to grow and divide resulting in groups of 

 colorless, thin-walled, clavate cells held together in clusters by a portion 

 of the undissolved gelatinized thick cell wall. 



Studie-s on -the time which intervenes between the application of 

 tlie stimulus and the abscission process as well as the time involved in 



