3o8 



"chlorosis," "alhication," "etiolation," are the most common names for the 

 condition in which a leaf loses its green coloring matter in spots, or over 

 the whole extent of its surface. The causes for this change in color are 

 very different, but always represent a condition of weakness. 



In order to survey the manifold causes of the disease, we will endeavor 

 to group them into 



I. Induced and non-transmissible conditions. 



(a). The discoloration attacks the whole surface of the leaf, 

 which has matured in the light. After having been green 

 in its young stages, the whole leaf assumes a yellowish, 



yellow to yellow- 

 white color tone. 

 Icterus or jauyi- 

 dicc. Cause : usu- 

 ally a lack of nu- 

 tritive substances, 

 (b). The pale discol- 

 oration is present 

 in the young organ 

 and the leaves re- 

 main in a con- 

 dition resembling 

 youth until their 

 p r e m a t ure end. 

 Cause : lack of 

 light and at times 

 of heat (see these 

 topics). 

 . Innate and transmis- 

 sible conditions. 

 Portions of the 

 plant show yellow 

 to pure white spots or stripes. Those plants suffer especially in 

 which pure white leaves appear near the ones spotted with green 

 or all green. The spots have usually a sharp demarcation. 

 White-leovedness, alhication, variegation, sometimes transmissible 

 through seeds or by grafting. Cause: probably enzymatic dis- 

 turbances (see these). 

 Of course there are intermediate stages between the types named, since 

 the individual causes often work together. 



In the present division we will examine only the icteric conditions and 

 treat them under lack of iron because, since the investigations of the Gris', 

 father and son, it is customary to consider jaundice as caused especially by 



Fig. 41. Buckwheat plant 

 free from clilorine. 



grown in a solution 

 (After Nobbe.) 



Gris, A., Ann. scienc. nat., 1875, VI ser. Vol. VII. p. 201. 



