334 Morphogenetic Factors 



decreased at upper levels. The leaves at different levels transpired at 

 about the same rate. 



Alexandrov, Alexandrova, and Timof eev ( 1927 ) observed that in 

 Bryonia (a running vine) the number of vessels in any part of the stem 

 varies with the dimensions of the leaves in that region. The size of 

 the vascular tissue in a petiole is also related to the area of its lamina. 



In fir, spruce, and beech, Huber ( 1928 ) found that the relative con- 

 ducting surface (the ratio of the area of conducting tissue to the fresh 

 weight of leaves above it) increases from base to apex in the stem. In 

 lateral branches this ratio was smaller than in the main stem, a fact 

 presumably related to the dominance of the latter. The relative conduct- 

 ing surface and the amount of transpiration were determined for various 

 plants by Huber and the rate of flow of water through the vascular sys- 

 tem calculated. This was found to be relatively high for herbaceous 

 plants, low for conifers and xerophytes, and intermediate for broad- 

 leaved trees, thus seeming to be related to the amount of transpiration. 

 Huber also observed ( 1924 ) that in oak branches growing in bright sun- 

 light a square decimeter of leaf area transpired 75 mg./hour and was 

 supplied through a cross-sectional area of 0.42 sq. mm. of vascular tissue. 

 In branches growing in shade there were 46 mg. of water transpired 

 through a vascular cross section of 0.20 sq. mm. In other words, the 

 greater the transpiration stream passing through the vascular system, 

 the larger this system was. He believes, however, that the amount of 

 water carried upward depends primarily on the osmotic pull exerted 

 by the leaves and the resistance to flow in the vascular tissues rather 

 than solely on the size of the vascular tissue itself. 



These various facts show that there is a definite correlation between 

 the amount of water passing through the vascular tissue and the amount 

 of such tissue that is developed. This, consequently, suggests that the 

 transpiration stream itself acts as a formative stimulus for the differentia- 

 tion of vascular tissue. Doubt as to the correctness of such a conclusion, 

 however, is raised by some remarkable results reported by Werner ( 1931 ) 

 in maize. He was able to grow a plant of almost normal size (suspended 

 in the air in a transpiration experiment) that was connected to its root 

 system in the soil by only a single extremely thin root about 10 cm. 

 long. The entire water supply for this plant, the stem of which was 

 several centimeters wide, passed through a vascular strand only about 

 0.5 mm. in diameter. Here, at least, there is little evidence that a heavy 

 transpiration flow stimulates a proportionate development of vascular 

 tissue. Similarly, in a frond of Osmunda, all the water transpired by a 

 large blade area is drawn through a leaf trace which is very small as it 

 leaves the stele. At the base of the frond this expands into a ring of 

 large bundles. 



