344 Varietäten etc. — Physiologie. 



n'y avait d'abord que deux variötös: Mexique et Tahiti. Le premier 

 type £tait considere" comme le Vanilla planifolia; quant au second r 

 d'aucuns pensent au V. pompona, mais sa determination n'avait 

 jamais ete faite d'une maniere precise. Les auteurs ont indiqud 

 que cette maniere de voir n'est nullement fondee et que le V. pom- 

 pona est une espece tres distincte et entierement differente. Les 

 deux types Tiarei et Haapape ont fait leur apparition sans qu'il y 

 ait eu aucune importation de nouvelles boutures dans l'archipel. La 

 presence du type Tiarei fut constate" il y a une dizaine d'annees; 

 le type Hapaape a fait son apparition depuis deux annees. 



Tous ces types sont considere's par les auteurs comme des 

 varietes du Vanilla planifolia Andrews et bien le type Mexique 

 comme V. planifolia var. sativa, les trois autres types sont des 

 sousvari£t£s angusta de V. planifolia var. sylvestris. 



M. J. Sirks (Wageningen). 



Shull, C. A., Measurement oft he surface forces in soils. 

 (ßotan. Gazette. LXII. p. 1—31. 5 Fig. 1916.) 



The results of these researches are summarized by the author 

 as follows: 



The force with which the seeds of Xanthium pennsylvanicwn 

 absorb water has been measured by two methods: a) osmotic Solu- 

 tion, and b) vapor pressure equilibrium. The osmotic method is at 

 present the more reliable. 



The air-dry seeds of Xanthium show an initial attraction for 

 water of nearly 1000 atmospheres. 



The attraction which exists at any moisture content oftheseed 

 between air-dry and Saturation can be approximated. Table III gives 

 the data. 



The seeds have in turn been used to measure the complex 

 moisture-holding forces of soils, with the following results: 



a) The air-dry subsoil of the Oswego silt loam holds its hygros- 

 copic moisture with about the same force as an air-dry seed, that 

 is, about 1000 atmospheres. 



b) As the moisture content of the soil increases, the surface 

 force decreases rapidly. When about 3,5 per cent of water has been 

 added to the air-dry soil, the force remaining is about 375 atmo- 

 spheres. When the soil moisture reaches 6 per cent above air-dry 

 in this soil, the moisture is held with a force of 130 or more atmo- 

 spheres. At 11 per cent above air-dry the holding power has fallen 

 to 22,4 atmospheres. 



c) At the wilting coefficient of the soil (13,3 per cent above air- 

 dry in the Oswego silt loam subsoil) the „black pull" of the soil 

 particles amounts to not more than that of a 0,1 M. NaCl Solution, 

 that is, not more than about 4 atmospheres. This is shown to hold 

 true for a number of types of soil with widely varying willing 

 coefficients. 



This water-holding power of soils at the wilting coefficient is 

 less than the osmotic pressure of the root hairs of many kinds of 

 plants, as shown by Hannig and others. 



The wilting of plants at the wilting coefficient of the soil cannot 

 be due to lack of moisture in the soil, nor to lack of a gradient of 

 forces tending to move water toward the plant. 



The view is held, therefore, that the wilting^ at this critical soil 

 moisture content must be due to the increasing slowness of water 



