The Dia-Heliotropic Attitude of Leaves. 



167 



from the epidermis to the central pith is found to rise suddenly to a maximum 

 in the phloem portion of the fibro -vascular bundle ; the xylem shows little 

 or no transmitted excitation. Hence we arrive at the conclusion that it is 

 the phloem which functions as the nerve of the plant. The characteristic 

 electric maximum was not found in experiments where the probe missed the 

 phloem ; greater experience now enables me so to direct the passage of the 

 probe as not to miss the nerve tract. 



In the diagram of the transverse section of the petiole of Mimosa usually 

 given in text -books there is in each bundle a single phloem strand outside 

 the xylem. I was, therefore, considerably puzzled by the fact that in 

 traversing the bundle I obtained two electric maxima, one before reaching 

 the xylem, and the second after passing it. In order to determine the cause 

 of this anomaly I made transverse sections of the petiole of Mimosa. 

 Differential staining clearly brought out the fact that the phloem strand is not 

 single but double, one above and the other below the xylem. The second 

 electric maximum coincided with the inner phloem. 



Fig. 6. — Micro-photograph showing a quadrant of the petiole and the fibro-vascular 

 bundle. The tissues seen in the section are : the epidermis, the cortex, the bundle 

 sheath, the first phloem, the xylem, the second phloem, and the central pith. 



