484 



DISEASES OF ECONOMIC PLANTS 



pores) and elaborated under the influence of sunlight and by 



the aid of the green substance (chlorophyll) into sugar or starch, 



, two of the fundamental 



substances upon which 

 plant growth depends. 

 Any injury or diminu- 

 tion of the green plant 

 part results at once in 

 diminished carbon .as- 

 similation (photosjmthe- 

 sis) and as a consequence 

 in lessened growth. Ab- 

 straction of elaborated 

 carbon or other useful 

 substances by parasites 

 is of course detrimental. 

 The starch or sugar made 

 in the green parts is 

 needed in any growing 

 part, especially in the 

 roots where none is made. 

 This necessitates root- 

 ward conduction, which 

 occurs through the bark 

 or through the bark por- 

 tion (phloem) of a vein 

 (Fig. 206). Injury to 

 the bark thus results in 

 stoppage of the downward sap movement and in retarded root 

 growth. 



Gaseous absorption (oxygen and carbon dioxide) is chiefly 

 through the stomata of the leaves and enlarged stomata (lenti- 

 cels) of the bark. Clogging of these openings as by dust, over- 

 growth by fungi or lichens, etc., interferes with this function. 



Fig. 204. — Experiment ahowing ascent of sap 

 current through wood. Bark removed does 

 not cause the leaves to wither beyond. From 

 Strasburger's textbook. 



