In very many cases a sickness which has already set in is indicated by 

 chlorosis, beginning inconspicuously and progressing slowly. Even if it is 

 possible to observe the very beginning of chlorosis, the beginning of the sick- 

 ness itself has in no way been discovered since the first molecular changes, 

 which have led to the yellowing of the chloroplast, still remain unknown to 

 us. The boundary line where any single factor of growth ceases to be bene- 

 ficial and becomes a retarding factor may indeed be determined experiment- 

 ally but in this we see only the final result and not the course of development ; 

 i. e., the processes initiating this final result. So far as our powers of obser- 

 vation are able to discover, health and disease represent conditions which 

 imperceptibly pass over into one another. 



3. The Relation of the Plant to its Environment. 



In the attempt, undertaken in the previous section, to demonstrate how 

 health and disease present interdependent conditions like the links of a chain, 

 we kept in view first of all the so-called constitutional diseases. By this are 

 understood the disturbances in nutrition which influence the whole organism 

 sympathetically and are the results of deficiency or excess of one of the 

 necessary vegetative factors. Local diseases due to accidental interference 

 must be opposed to these general diseases. In them the organism as a whole 

 in its full reactionary capacity is exposed primarily to a disturbance affect- 

 ing only one individual organ. While the action of the necessary inorganic 

 factors of growth come under consideration in constimtiona! diseases, in 

 local diseases the important influences arc those mutually exerted on one 

 .another by the organisms. 



There are insects which seek out the plants iii order to satisfy their 

 needs for nutrition or for habitation, or the planis themselves mutually in- 

 ' fiuencc one another. We find as the most pertinent example the influence of 

 street trees on the plants growing on the other side of the hedge row. We 

 notice especially in times of drought that the grain and potato plants found 

 within reach of the tree's shadow are not only weaker in development but 

 wilt sooner and to a greater degree than the other plants in the same field. 

 This disadvantage is due chiefly to the tree which keeps of? the rain and its 

 roots which withdraw the soil water. In the field itself we frequently find 

 different places in which the seed has grown very poorly because the wind 

 grass has choked the grain. The seed was not sown too thin but the germi- 

 nation and first development were choked by cold and deficiency in oxygen 

 because of impervious spots in the field. In spring the soil does not dry so 

 quickly in these places and the moisture is retained longer; the soil conse- 

 quently warms up less easily and suffers for need of oxygen. The wind 

 grass (Apera spica venti) which occurs everywhere in grain fields is less 

 sensitive and under such conditions develops more quickly than grain. 

 Because of its greater size, it chokes out the seedling grain. Similar con- 

 ditions arise in connection with other weeds, which, developing more rapidly, 

 not only take food materials out of the soil and away from the cultivated 



