DoYLK — Some Researches in Experimental Morphology . 437 



neuer Trieb mit Laubblattern eine neue Holzzone niit weiten Gefasseii 

 beginnt. Zweckmassig ist jedenfalls dieses ZusammeiitrefFen denii mit dem 

 neuen Laiib tritt erbohte Transpiration ein, zu deren Deckung weiteie 

 Wasserbahuen uotig sind." The very same remarks might, indeed, apply lo 

 all the dry-air experiments, and to our own petiole. Again, Winkler makes 

 an appeal to ecology. Desert plants have, he says, a muoli reduced vascular 

 system, due to a reduced transpiration as a result of protective devices. Tliis 

 is a somewhat startling generalization, but it allows one point to be made with 

 regard to the ecological aspect of transpiration. For the literature seems 

 very poor indeed with regard to such points as comparisons between the 

 absolute transpiration of Xerophytes and desert plants, and that of tempe- 

 rate mesophytes, as well as any statistics as to the relation existing between 

 the transpiration of xerophytes and the woody development in them. 



A critical exposition of such would much aid us in our inquiry into the 

 causes of the development of secondary wood. We are often inclined to look 

 upon Xerophytes as plants whose transpiration is absolutely small. If so, we 

 find — in spite of Winkler's opinion above — that " in contrast to Hydro- 

 phytes, most Xerophytes and alpine plants have highly developed conductive 

 systems with large, thick-walled elements." (Oowles 44, p. 686.) Schimper 

 (42, p. 349), too, says that "the majority of trees of xeropliilous woodland 

 (i.e., in tropical rain forests with dry seasons) and savannah are of low 

 stature with a relatively thick stem ; the volume of wood in comparison with 

 the foliage is greater tlian in hygrophilous trees." And again (p. 612) : 

 " Desert plants, dependent upon subterranean water, mostly have lignified 

 axes." When, however, we remember witli Delf (45 and 46) that many 

 succulents — especially halophytes — have a high transpiration, i.e.. Trans- 

 piration of Salkornia bears to that of Vicin tlie ratio of 36 to 19 in one case ; 

 when we remember with Holterman (47) that the transpiration of plants in 

 the tropical Ceylon forest may equal, if not exceed, that of temperate meso- 

 phytes, and with Schimper (42, p. 308) the brittle structure with a few vessels 

 of herbaceous plants of the tropical rain forest, then we feel the need of that 

 critical discussion on vascular development from the ecological standpoint. 

 But we may mention that Cowles (44) seems largely of the opinion that 

 desiccation, whether brought about by increased transpiration or by 

 diminished absorption, appears to stimulate increased vascular development, 

 the important point being a relatively great transpiration to absorption. 



What it is sought to convey in the last few paragraphs is that tlie foliage 

 development bears some large part in the production of the secondary 

 tliiekening in our petiole ; that transpiration, or rather the water change in 

 the plant, is the most probable factor ; that, with our incomplete knowledge, it 



SOIENT. PEOC. R.D.S., VOL. XIV., NO. XXXIII. 4 A 



