The Dia-Heliotropic Attitude of Leaves. 



169 



proviug that it was a non-conductor; when the probe reached a depth of 

 0'35 mm. it encountered the second phloem, where the response underwent a 

 second enhancement of —56 divisions. The probe reached the border of 

 the pith at a depth of 0*4 mm. and the response underwent a diminution 

 to —26 divisions. In cases where the incident stimulus on the sub-petiole 

 is feeble the irradiation effects are greatly diminished ; the excitatory trans- 

 mission is then found only in the phloem. I give below a summary of results 

 obtained with ten different specimens : — 



Table I. — Showing Intensity of Transmitted Excitation in Different Layers 



in Ten Different Specimens. 



Different 



Transmitted excitation. 



layers. 

























I. 



II. 



III. 



IV. 



V. 



VI. 



VII. 



VIII. 



IX. 



X. 



Mean. 



Epidermis... 



+ 1 



























+ 4 











+ 0-5 



Cortex 



-2 



-3 



-50 































-5 5 





-30 



-30 



-100 



-30 



-36 



-44 



-33 



-18 



-20 



-24 



-36 -5 





-8 



-9 















-10 







-4 



-8 



-8 



-4-7 





-30 



-30 



-84 



-10 



-36 



-20 



—12 



-18 



-20 



-16 



-26 '8 



Pith 







-6 



-29 











-7 



















-4-2 



It will be seen that in all eases the phloem is invariably found to be the 

 best channel for conduction of excitation. The following curve (fig. 8), 



Layer of cells . 



Fig. 8. — Curve showing the different intensities of transmitted excitation in different 

 layers : E, epidermis ; C, cortex ; P, first phloem ; X, xylem ; P', second phloem ; 

 O, pith. 



VOL. XCIII. — B. 



