Doyle — Some Researches in Experimental Morphology. 436 



(27 and 28), Snell (30), Winkler (41), aud Soberer (43) have shown that the 

 further development of the leaf-trace depends on its intimate connexion 

 with the developing-leaf. " Organbildung," says Jost, " ist in vielen 

 Fallen eine notwendige Bediugung fur die Gefassbildung." Snell and 

 Winkler have also shown that it is not necessary to remove the leaf — if its 

 function is removed, we have the same result. Again, secondary thickening 

 in trees seems to go normally baud in hand with leaf-development. This is 

 most strikingly seen in those of our common trees which may bear 

 " Johnnistriebe " — that is, branches upon which the buds suddenly open and 

 develop in late summer. When this happens, a second yearly ring is 

 commonly seen (Jost. 27). Jost himself cannot bind himself to anything 

 more definite than the statement that it is a correlation ; but Winkler 

 plumps for transpiration and, presumably, " der Grad der Inanspruohnahme 

 der Gefasse " as the factor. Now there is much evidence to show that 

 a general connexion exists between transpiration and vessel-development. 

 The greater quantity of vascular tissue in plants grown in dry air than in 

 moist, the greater vascular reduction of water-plants, may all be adduced in 

 favour of the contention. (For references to a literature which it is 

 unnecessary to quote here, see Winkler (5), and Schimper (42)). 



But admitting a general connexion between transpiration and vessel- 

 development, we have no accurate knowledge of tlie closeness of this con- 

 nexion. In the first place, quantitative measurements on any extended scale 

 seem lacking in the experiments referred to above. We do not know in 

 most cases by how much the transpiration increased in the dry chambers, 

 still less how far the vascular increase bore any close relation to such 

 transpiration increase, nor do we know the relations between transpiration 

 and absorption. Again, assimilation and respiration increase also in dry air 

 and light, and the effects of such metabolic changes, acting with transpira- 

 tion, have never been analysed ; indeed, it is difficult to see how they could 

 be. Again, granting that transpiration is the factor, what evidence have we 

 for the " Grad der Inanspruohnahme " theory ? Jost (24, p. 430) says in 

 this connexion : " Bei starker Transpiration wird die Kutikula verstarkt, 

 Kollenchym und Sklerenchym gefordert, die Gefasse werden weiter und 

 zahlreicher, in den Blattern tritt reiohlich Palisadenparenchym auf. Es 

 teldt aber zurzeit noch an einer kritischen Untersuohuug der gauzen Frage, 

 wir wissen nicht, wieviel von den beobachteten Erfolgen einfach auf Kosten 

 von Dittereuzen im Wassergehalt der Pflanze, wieviel auf Verschiedenheiten 

 in der eigentlichen Transpiration zu setzen ist, und im letzteren Fall ware 

 weiter zu untersuchen, ob die Wasserabgabe als solche einen Eeiz ausiibt oder 

 ob die mit der Transpiration in naher Beziehung steliende Yersorgung 



