INTEUCELLULAlt SPACES. 



99 



sj-stem. To this system should perhaps be referred also numer- 

 ous easee of pigment-cells, like those in the roots of madder and 

 rhubarb ; also the peculiar bodies seen in tlie periphery of the 

 pith of Sambuc-us, and the millc-sacs of some species of Acer. 



297. 3Iucilage-cells are lai'ger than the surrounding cells, and 

 sometimes closel3- resemble intercellular spaces filled with muci- 

 laginous matter. In some instances the mucilage is distinctl}- 

 referable to changes in the contents of the cell, in others to a 

 disorga)iizatiou of a portion of the wall, while in still others 

 both sources may be recognized.^ 



298. Cells containing tannin in very large amount are fre- 

 quently met with, but they do not call for special remark. 



299. Resins and the like arc found not only in single cells 

 but also in spaces formed by the breaking down of the interven- 

 ing wall j of cell-clusters of various shapes ; hence various forms 

 of receptacles for these substances may be looked for. 



INTEIiCELLULAR SPACES. 



300. The walls of cells still capable of division are generally in 

 unbroken contact ; but as difterentiation goes on they may be- 

 come separated more or 

 less by unequal growth 

 or hy a breaking down 

 of intermediate cells. ^ 

 The intercellular spaces 

 thus formed ma}' he mere 

 chinks, or they ma}- be- 

 come chambers of large 

 size. They may con- 

 tain merely air, or air 

 and water}- sap, or most 

 of the matters described 

 in the previous sections. 



Air-spaces in the 

 looser tissues of plants 

 are generally so con- 8i 



1 The details of this subject can be found in Prings. Jahrb., v. 161 (Frank), 

 and Anuales des Sc. nat., sev. 6, tome i. p. 176 (Prillieux). 



^ The first mode of development of intercellular spaces has been termed 

 scMxogenic, the latter lyaigenic ; moreover, a distinction may be made between 

 those intercellular spaces which are formed when the tissues begin to difTereu- 

 tiate, — protogcnic, — and those formed in older tissues, — hysterogenic. 



Fio. 81. Transverse pectimi tliroiigli the stem of Elatiue .llsiuastrum, showing large 

 intercellular spaces, /i, eoiitaiiiing air. (Reinke.) 



