470 THE FORMS OF TISSUES [ch. 



has become celebrated in botany under the name of Sachs's Rule, 

 that one cell- wall always tends to set itself at right angles to another 

 cell- wall. But this rule only applies to the case we have just 

 illustrated; and such validity as it possesses is due to the fact that 

 among plant-tissues it commonly happens that one cell-wall has 

 become soUd and rigid before another partition- wall impinges upon it. 

 (5) Another important principle . arises, not out of our equations 

 but out of the general considerations which led to them. We saw in 

 the soap-bubble that at and near the point of contact between our 

 several surfaces, there is a continued balance of forces, carried (so 

 to speak) across the interval; in other words, there is physical 

 continuity between one surface and another and it follows that the 

 surfaces merge one into another by a continuous curve. Whatever 



A^ 



Fig. 154. Plateau's bourrelet, 



in an algal filament. After a b 



Berthold. Fig. 155. 



be the form of our surfaces and whatever the angle between them, 

 a small intervening curved surface is always there to bridge over 

 the Hne of contact; and this Uttle fillet, or "bourrelet," as Plateau 

 called it, is big enough to be a common and conspicuous feature in 

 the microscopy of tissues (Fig. 154). A similar "bourrelet" is 

 clearly seen at the boundary between a floating bubble and the liquid 

 on which it floats: in which case it constitutes a "masse annulaire," 

 whose mathematical properties and relation to the form of the 

 nearly hemispherical bubble have been investigated by van der 

 Mensbrugghe*. The superficial vacuoles in Actinophrys or Actino- 

 sphaerium present an identical phenomenon. 



(6) It is a curious efl'ect, or consequence, of the bourrelet that 

 a "horizontal" soap-film is never either horizontal or plane. For 

 the bourrelet at its edge is deformed by gravity, and the film is 

 correspondingly inclined upwards where it meets it (Fig. 155 6). 



* Cf. Plateau, op. cit. p. 366. 



