ACCESSORY VASCULAR BUNDLES 381 



embody the same governing principle, namely, that which aims at 

 securing for every organ of the plant body a uniform and uninterrupted 

 supply of the nutritive materials necessary to its growth and activity. 

 As already explained, this co-operation among the various leaf-traces 

 prevents inequalities in the incidence of external factors of nutrition 

 from inducing an asymmetrical development of the plant-body, and 

 thus ensures that the normal plan of construction is carried out, in 

 spite of the frequent occurrence of such inequalities. 



For the sake of greater clearness, we may consider an imaginary 

 case in which a single bundle enters the stem from each of the 

 numerous leaves, and then runs down perpendicularly without forming 

 any connection with other leaf-traces. This arrangement would, in a 

 sense, represent the most complete contrast to the type of vascular 

 system that consists of a simple axile strand. If now the flow of 

 water and nutrient salts were to be interrupted on one side of our 

 imaginary stem, owing to some local injury, all the leaves and axillary 

 buds on that side would inevitably wither and die, and the entire plant 

 would thus suffer a serious loss. The scheme of vascular arrangement 

 which actually prevails, enables the organism to react as a co- 

 ordinated whole towards the various external agencies that tend to 

 interfere with translocation ; as a result, the plant can deal more 

 effectively with inequalities in the flow of food materials, and can 

 even suffer local disorganisation of the vascular system without serious 

 inconvenience. 



Modifications of the normal Dicotyledonous type of vascular system 

 arise in a great many different ways. A very frequent abnormality 

 consists in the presence of numerous accessory medullary bundles, which 

 may represent leaf-traces that have penetrated very far into the stem, 

 or which may, on the contrary, be independent cauline strands. The 

 former case is exemplified by most Cucurbitaceae, by the Piperaceae, 

 and by species of Palaver, Thalirtrum, Actdea, etc., the latter by 

 various species of Begonia, Aralia, Orobanche, by certain Melasto- 

 maceae, Umbelliferae, etc. The presence of accessory vascular 

 strands in the cortex, i.e. outside the circle of ordinary bundles, 

 is on the whole less frequent. Such cortical bundles may merely 

 represent loops or branches of ordinary leaf-traces (Lathyrus Aphaca, 

 L. Pseudaphaca, Casuarina, Salicornia, the Cactaceae, many species 

 of Begonia), or they may constitute an independent cortical system 

 of leaf-traces (Calycanthaceae and many Melastomaceae). Accord- 

 ing to Heinricher, finally, the cortical strands that occur in many 

 species of Centaurea are in all probability genuine cauline bundles. 

 The few detailed investigations which have hitherto been carried 

 out upon the subject, clearly show that these " anomalies " of vascular 



