298 



THE FORMS OF TISSUES 



[CH. 



as Plateau called it, is large enough to be a common and con- 

 spicuous feature in the microscopy of tissues (Fig. 102). For 

 instance, the so-called "splitting" of the cell- wall, which is con- 

 spicuous at the angles of the large "parenchymatous" cells in the 

 succulent tissues of all higher plants (Fig. 103), is nothing more 

 than a manifestation of Plateau's "bourrelet," or surface of 

 continuity*. 



We may now illustrate some of the foregoing principles., 

 before we proceed to the more complex cases in which more 

 bodies than three are in mutual contact. But in doing so, we 

 must constantly bear in mind the principles set forth in our 

 chapter on the forms of cells, and especially those relating to the 

 pressure exercised by a purved film. 



Fig. 102. (After Berthold.) 



Fig. 103. Parenchyma of Maize. 



Let us look for a moment at the case presented by the partition- 

 wall in a double soap-bubble. As we have just seen, the three 

 films in contact (viz. the outer walls of the two bubbles and the 

 partition-wall between) being all composed of the same substance 



the partition -walls themselves are composed, is obviously tending to a reduction 

 of the internal surface-area. And it may be that it is as well, or better, accounted 

 for on this ground than on Plateau's assumption that it represents a "surface of 

 continuity." 



* A similar "bourrelet" is admirably seen at the line of junction 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 {cf. 

 Plateau, oj). cit., p. 386). The form of the superficial vacuoles in Actinophrys or 

 Actinosphaerium involves an identical problem. 



