i 5 o SCIENCE PROGRESS. 



immediately upon the peridesm of the bundle, specially 

 characterised. The cells of the phloeoterma are often de- 

 prived of chlorophyll, or this is confined to the side walls, 

 and these walls may also be suberised. It is, however, a 

 rare case for such layers to be united in a continuous 

 system with the phloeoterma of the stem, and thus to shut 

 off completely, by means of a continuous membrane, the 

 entire stelar system of the plant from its cortical tissue. 

 This state of things obtains, however, in Pinus and some 

 dicotyledonous genera, e.g., Galium. In most dicotyledo- 

 nous petioles endodermoid layers, if distinguishable at all, 

 are often incomplete and not necessarily formed from the 

 phloeoterma. A closed sheath to the bundles is, however, 

 often formed in Angiosperm petioles by thickened peri- 

 desmic (stelar) tissue, such a sheath being called by Stras- 

 burger a stelolemma (15). The ensemble of the phenomena 

 shows us, clearly enough, that the endodermis, in its original 

 sense, cannot be taken here, any more than in the stem, 

 as a layer of constant morphological value. The phloeo- 

 terma may be distinguishable by endodermal or other char- 

 acters, but on the other hand, it may not. 



The main fact in regard to the vascular system of the 

 leaf is one which was pointed out by Van Tieghem in 1870. 

 The system is bilaterally symmetrical in relation to the 

 plane including the organic axes of both leaf and stem, 

 and not, like that of root and stem, radially symmetrical 

 about its organic axis. The designation of the continuous 

 cylinder of root and stem as a stele and of each bundle or 

 the whole bundle system of the leaf as a schistostele or 

 meristele is in complete accord with this general fact. But 

 we must not disguise from ourselves that both the stele and 

 the meristele may not exist in the adult as sharply separated 

 structures. 



A. G. Tansley. 

 ( To be contimted. ) 



