240 MACROMOLECULAR COMPLEXES 



and other agencies. It has been shown by Handley (1954), however, 

 that, under the same conditions, leaves which have died on the plant 

 behave very differently. Although the vascular bundles largely dis- 

 appear, the cells of the epidermis and mesophyll, which might have 

 been considered much less resistant, remain almost intact. Dealing 

 with mor soils, Handley remarks, "Serial sections (of recognizable 

 fragments of litter of Calltma vulgaris— present author) indicated 

 that vascular tissue disappears first, leaving a residue of leaf meso- 

 phyll tissue which ultimately becomes an amorphous mass contain- 

 ing mesophyll cell walls apparently coated with some protective 

 material. Similar findings were obtained for . . . beech litter, Nor- 

 way spruce litter and Abies pinsapo litter. . . . From serial sections 

 of shoots of Calluna vulgaris, having both dead and living leaves, 

 changes have been observed in the mesophyll tissue when the leaf 

 dies, while still on the living plant, which give the mesophyll tissue 

 the appearance it retains until it becomes amorphous material in the 

 mor. The material appearing . . . would seem to be that which 

 protects the cellulose wall. . . . Experimental evidence is put for- 

 ward to show that the essential part of the change occurring when 

 the leaf dies is a precipitation or stabilisation of cytoplasmic pro- 

 tein." 



From this point of view, the recent findings of Steward and Pol- 

 lard (1956, 1957) and Pollard and Steward (1959) are not without 

 significance. They have shown that, when cultures of carrot-root 

 phloem and potato tubers are fed with C^^-labeled proline, the pro- 

 line can be recovered in a protein which also contains C^^ hydroxy- 

 proline, clearly produced from the proline. Apart from this, the 

 active carbon appears in no other amino acid, nor does it appear as 

 COo. The protein concerned is therefore stable and does not take 

 part in the normal protein turnover in the plant. Moreover, the 

 protein is not part of any of the cytoplasmic particles which Steward 

 and Pollard examined, and they conclude, therefore, that this pro- 

 tein occurs in the ground cytoplasm. Here, then, is a protein of 

 which at least the proline- and hydroxyproline-containing portion 

 is guarded from the metabolic machinery of the plant. One possibil- 

 ity is that this is the protein which is closely associated with the 

 developing cell wall. This is the only protein extracted from plants 

 to contain hydroxyproline, and recalls immediately the composition 

 of the protein in the cellulose-protein complex described by Hall, 

 and discussed in an earlier section of this paper. 



