1918] Kendall: Abscission of Flowers and Fruits in Solaimceae 357 



enzyme of some kind is the active factor. Indeed, since all liydrolys- 

 ing processes of living cells are now supposed to be due to the action 

 of enzymes, there is no reason to suppose that the hydrolysis of the 

 middle lamella does not conform to the general rule. For it is known 

 that an enzyme, pectosinase, is capable of breaking down the pectose 

 of which the middle lamella is composed. However, until more is 

 known concerning the nature of this particular enzjane it remains 

 impossible to get more definite evidence on this phase of the problem. 



3. Abscission of the Corolla 



Reiche (1885) gives an account of the fall of the corolla in a 

 large number of species belonging to about forty-five families of the 

 monocotyledons and dicotyledons. He finds that the corolla may be 

 thrown off in one of three different ways : ( 1 ) by the activit}' of a 

 small-celled separation layer; (2) through decay; (3) through in- 

 crease in size of the ovarj^ thus tearing off the tissue involved at 

 the base of the corolla. In many cases of true abscission — case 1 

 above — Reiche finds that the separation layer is preformed and ready 

 to function at any moment. This represents a contradiction of 

 Mohl's observations, according to which the fall of the corolla is 

 usually due to the action of a separation layer formed shortly before 

 fall. According to Reiche, the separation layer is very seldom morpho- 

 logicall}^ differentiated from the neighboring tissue, but in a few cases 

 he describes the separation layer as consisting of a layer of cells 

 smaller than the neighboring cells on either side. 



Kubart (1906), in his account of abscission of the corolla in sev- 

 eral different species, describes and figures the process which takes 

 place in Nicotiana. The separation layer in this genus he finds to be 

 in no way morphologically differentiated, of indefinite shape, and 

 located about 1 mm. above the base of the corolla tube. In this gen- 

 eral region a large number of cells separate from one another, all the 

 cells in cross-section taking part except the epidermal cells and the 

 tracheae. Fitting (1911), in his work on the shedding of petals, de- 

 scribes the process of abscission in several genera, paying particular 

 attention to Erodium, Geranium, Linum, Helianthemum, Perlagmiium, 

 and Verhascum. Separation in these cases takes place through a 

 region of small, spherical cells rich in protoplasm. The separation 

 layer is not sharply differentiated as compared with the tissues on 

 either side but is located in a restricted region at the base of the petal. 



