Doyle — Sotne Researches in Experimental Morphology . 4lo 



developed along the whole length of what we may term tlie " xylem-free " 

 surface of the phloem, and even formed a few isolated vessels on this side 

 (Plate XXXI, fig. 1). 



These small vessels are sliown at ('/). This is the only instance of the 

 appearance of lignified tissue or conducting vessels of any sort on the 

 xylem-free surface of the central bundle out of more than 50 petioles 

 examined. Plate XXX, fig. 4, is the type of the rest. "We may mention, 

 too, that the grafts on this petiole, A 1386 and A 1374, above, botli died as 

 a result of their atmosphere having been kept too warm and moist. 



Plate XXXI, fig. 4, shows tlie further fact we must learn, that the central 

 bundle does not always remain one, but in many petioles is found split, 

 Thus («), (6), (c) all constitute tlie central bundle here. When, however, we 

 remember that when coming from the leaf lamina the central bundle is formed 

 nominally from the fusion of several bundles, this appearance is not surprising. 

 Indeed, if we follow the central bundle down along the petiole, we find very 

 frequently that the bundle will split and unite, and then split again a couple 

 of times. The phloem is remarkably well shown indeed in (6) — its demarca- 

 tion from the cortex almost gives one the impression of its being suiTounded 

 by a bundle sheath. 



We see then that the detailed structure of the petiole is very constant and 

 differs from Jaunicke's description in the following points : — 



1. Wood is developed on the under-side of the central bundle only, and 

 not on the upper-side as well. 



2. There is no complete ring of cambium in the central bundle, 



3. There is no " bast " formed within that cambium. 



4. A large quantity of phloem is formed. 



These divergencies from Jannicke seemed so great as to require further 

 investigation. And so it was considered necessary to look into this point just 

 a little, although its connexion with the taking on by the petiole of the 

 function of stem is meagre. 



And this for two reasons — 



1. Because Jannicke does not describe what variety he used. 



2. Because the petioles A 1374 and A 1386 gave one the idea that 

 perhaps the tendency to form a complete ring of wood was latent in the 

 petiole — a sort of Mendeliau recessive perhaps. 



These combined suggested that meteor was conceivably a hybrid between 

 the form used by Jannicke and a form which never developed cambium on 

 the xylem-free side of the central bundle — the latter condition being 

 dominant. 



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