28S DISEASE IN PLANTS. 



considerable ranges of water supply, concentration 

 of salts, partial pressure of oxygen, etc., implies that 

 the molecular activities of the protoplasm are of 

 the normal kind all the time, though they may 

 differ in rapidity, and even in quantitative and 

 qualitative respects within certain limits ; and the 

 meaning of the optitnum temperature, illumination, 

 oxygen pressure, etc., is, from this point of view, 

 not that the molecular activities differ in kind 

 from those nearer the minima and maxima, so 

 much as that they are running at the best rates 

 for the welfare of the plant i.e. for permanent 

 health. 



If we transcend the cardinal points limiting the 

 range of this play, however, and we get variations 

 in the kind as well as rates of molecular con- 

 structions and disruptions, then we pass by imper- 

 ceptible gradations into ill-health i.e. Disease. 



And similarly in relation to other protoplasm. 

 That of the right kind of pollen grain from another 

 plant of its own species, stimulates the contents 

 of the ovule to produce a vigorous embryo and 

 healthy seedling : that of a similar pollen grain in 

 its own flower either does no positive harm, but has 

 a feebler effect, or it may act like a poison. That 

 of another pollen grain again may refuse to unite at 

 all ; while that of a fungus hypha e.g. of Sclerotinia 

 on Vacciniuvi may run down the style as does 

 the pollen tube and produce death and destruction 

 throughout the ovule. 



Or again, in Clover, we may have the hypha of 

 a Botrytis with its protoplasm unable to do more 



