PATHOLOGY 133 



and for what kind of pollen chemotropism first becomes appar- 

 ent. The experiment can be varied by placing the stigmas of 

 one of the flowers at the center instead of the crystal of sugar. 

 It will attract some of the kinds of pollen tubes and have no 

 effect on others. (The pollen grains and their germination 

 can be observed much more easily if, in place of the filter paper, 

 the following be used: To a measured quantity of boiling culture 

 solution, sift in with constant stirring enough agar powder to 

 make a 2 per cent, solution. When thoroughly dissolved, pour 

 it into petri dishes and cover, and allow to cool. On the jelly- 

 like mass thus produced the germination of pollen grains can 

 be observed very easily.) 



(q) In the spring bring into the laboratory buds of tulip or 

 crocus, just about to open. In the warmer air they will soon 

 open by increased growth on the inner surface of the bases of 

 the petals and sepals. When fully opened, place in an ice-box 

 or out-of-doors on the window ledge, and very soon increased 

 growth on the outside will cause them to close. 



(r) Observe growing plants of sunflower (Helianthus), lamb's 

 quarters (Chenopodium), etc., by day and by night, and 

 notice the different leaf positions assumed by the younger 

 leaves. The fully developed leaves will show little or no change 

 of position. 



194. Pathology is the study of the abnormal develop- 

 ment and functioning of a plant. It is in its widest as- 

 pect abnormal physiology. As usually studied, however, 

 it is the determination of the cause of and means of pre- 

 vention of certain plant diseases. Since most plant 

 diseases that have been studied are caused by fungi, 

 pathology as taught is often but a study of mycology, in 

 which parasitic fungi alone are considered. These views 

 of pathology are in reality only partial views, and do not 

 take the real scope of the subject into consideration. 



195. Since abnormal functioning often leads to abnor- 

 mal structural development it is necessary to study not 

 only the abnormal functioning of a plant but also the 

 abnormal structures produced by the diseased conditions. 

 Thus we can distinguish cases in which cells or tissues do 



