INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 477 



two causes : — viz. (1) external or internal forces acting directly on the 

 increase of the cell-walls, accelerating the gx-owth on the convex, and 

 retarding it on the concave side ; (2) an increase of the osmotic ten- 

 sion between the cell-wall and cell-contents on the convex side. 

 In a communication to the Eoyal Academy of Amsterdam, H. de 

 Vries discusses these two theories, and gives the following as his 

 own conclusions : — When multicellular organs undergo curvature 

 in the course of their growth, the osmotic tension of the cells is 

 first modified, and tlie change in the rapidity of growth of the cell- 

 walls must be regarded as a secondary phenomenon. 



Movements of Air and Sap in the Living Plant.* — Dr. F. R. 

 von H()hnel has been engaged during the past three years on a very 

 extensive series of experiments with a view of deterniiuing several still 

 unsettled questions relating to this subject. Among the numerous 

 interesting conclusions at which he has arrived, the following are 

 some of the more important. 



In relation to the supposed direct communication between tlie 

 vessels and tlie stomata, his results go to show tliat no such communi- 

 cation exists; and he considers that he has also disproved Hales's theory 

 of a communication between the lenticcls and the bark. 



With regard to the difl'usion of air through plants, and tlie rela- 

 tion of this to the external pressure, Von Hohnel states that when 

 a negative pressure of air, or suction, takes place in a vessel from which 

 water has been removed, this pressure must not only descend into the 

 vessel, but must also spread laterally in both the radial and tangential 

 directions. Hence it follows that the powerful suction which takes 

 place in the vessels of the youngest annual ring must not only be more 

 or less uniformly distributed within the ring, but must also spread to 

 the older ring next within it. From the second and third rings water 

 must constantly be sucked up into the first, in which it rises and 

 l)asses away through transpiration. But since the inner rings contain 

 much air in their cells and vessels from the previous year, the negative 

 pressure in them can never attain the possible maximum for the j)lant 

 by the removal of water, and must therefore always remain consider- 

 ably less than in the youngest ring. It is, however, clear that in the 

 second ring the negative pressure must be greater than in the third, at 

 least when transpiration is active, since the water must first be removed 

 from it. 



Branching of Dorsiventral Shoots, f — Goebel points out that tho 

 origin of the prevalent theory of the external conformation of 2)laiits 

 rests on two hypotheses : firstly, tho s])iral theory ; and secondly, that 

 of axillary branching. 



If the si)iral theory were universally true, it would follow as a 

 ncccKSiiry conHctiucncu that the plant would jji-oiluce shoots uniformlv 

 on all siiles ; and such a mode of branching might bo termed rnilinr. 

 It is certain, however, that all modes of branching cannot be referred 

 to the radiar, as in those cases where there is a contmst not only 



• Mnhrh. wis.s. Hot.,' xii. (187!)) \h 47. 



t 'Arbeit, liot. Inst. Wiirzlmrf,',' ii. (ISSO) p. a.")!! 



