37O ELEMENTARY ORGANS. CHAP. III. 



SUBSECTION II. 



Gaps. Gaps, according to M. Mirbel, are empty, 

 but often regular and symmetrical spaces formed in 

 the interior of the plant by means of a partial dis- 

 ruption of the membrane constituting the tubes or 

 Perhaps utricles. They seem to be occasioned by the super- 

 r _ abundance of the nutritive juices which the vessels 



of sap n e are sometimes found to contain without being able 

 to elaborate, and by which they are ultimately 

 ruptured. They do not occur often, except in 

 plants of a soft and loose texture, such as aquatics ; 

 though they are sometimes to be met with even in 

 woody plants also. 



Yet some- In their general aspect they resemble longitudi- 

 jnetdSy" na ^ tubes interspersed throughout the cellular tissue 

 arranged. or p u ]p 5 as may be seen in the different species of 

 Potamogeton, or in the rachis of Ferns. But in 

 the Equisetum they assume a regularity of disposi- 

 tion that seems to indicate something more than 

 merely the accidental rupture of the vessels. One 

 gap larger than the rest occupies the centre of the 

 stem, around which a number of smaller gaps are 

 placed in a circular row, which is again encircled 

 with a second row of gaps larger than the last, and 

 alternating with them, and forming in their aggre- 

 gate assemblage a sort of symmetrical group. In the 

 leaves of herbaceous plants the gaps are often inter- 

 rupted by transverse diaphragms formed of a portion 



