IQO 



The Living Plant 



sugar molecules, may permit the passage of the much smaller 

 mineral molecules. But aside from this, the evidence shows that 

 in protoplasmic membranes another influence comes into play, 

 and that the dissolved substance, in order to pass through such 

 a membrane, must be soluble in the material composing it. 



There remains to be considered the absorption of gases, a matter 

 of great importance because of the indispensable part played in the 



plant's economy by both car- 

 bon dioxide and oxygen, the 

 great reservoir of which is 

 the air. The first requisite, 

 of course, to gas absorption 

 by the living cells, the most 

 of which lie deeply buried 

 within the body of the plant, 

 is some system whereby those 

 gases can be conveyed from 

 the atmosphere into their 

 presence; and such a sys- 

 tem, as the reader already 

 FiG^ 65.-A cluster of cells in a piece of pith, j^ learned iu Chapter II, is 



showing the intercellular air passages (in ^ ' 



black). (Copied from a wall diagram by provided in the iuter-cellular 



Frank and Tschirch.) . 



air passages, which are shown 

 in a typical tissue, a bit of pith, in the accompanying picture (fig- 

 ure 65). These passages do not exist in young tissue where new 

 cells are in process of formation, as figures 53 and 139 C illustrate; 

 but as the young cubical cells grow larger, they tend to round off 

 into spherical form, splitting in their mid-walls, first at the 

 angles and then along the edges, until the final arrangement 

 tends to approximate to that of the spaces and passages existing 

 between balls in a pile. These passages once formed always 

 persist, no matter what shapes the (sells may assume; and there- 

 fore they form a continuous system ramifying everywhere 

 throughout the plant, as is represented diagrammatically in the 



