MICHIGAN ACADEMY OF SCIENCE. 89 



by fungus hypluie or bacteria Avhich had begun to use the substance ex- 

 creted at or near the tip. We can therefore, see how, what seems to be 

 a waste substance, may prove of great advantage to a phint, and that 

 at a time of life when great demands are soon to be made upon nutrient 

 soil solutions. 



Excretions from leaves: During times of high activity in plants, ex- 

 cretions occur upon leaves as a result of checked transpiration ac- 

 companied by normal absorption. During the day time, iu land plants 

 provided with stomata a great deal of Avater is transpired in the form 

 of water vajior. During the night root activity is as great as in the day 

 time and trans])irati()n is reduced. The plant is provided with a 

 mechanism to relieve the pressure by excreting liquid matter. The chief 

 part of this mechanism is the water-pore, the morphological equivalent 

 of the stomate, though not its jihysiological equivalent. Stomates are 

 essentially organs concerned with transfer of gases. Water-pores are 

 concerned with the transfer of water (or solutions). A water-pore is 

 therefore, a sort of safety value and is vein- common among higher 

 l)lants. 



Solutions exuding from water-pores are found to contain organic and 

 inorganic matter. If these drops of solution fall to the ground, as they 

 sometimes do, the plant loses substance to the soil. If conditions be fav- 

 orable, much of this material may be resorbed through the water-pores 

 and taken in again into the cells of the neighboring tissues. If the exter- 

 nal air be dry it will cause a rapid evaporation of the drops of excretion 

 substance, and an incrustation will be formed in the region in the neigh- 

 borhood of the water-poi'e. 8ucli incrustations may be seen on wheat, 

 oats, barley and the like, on the tips of the leaves. In broad leafed plants, 

 they are found, on the margin near the ends of the veins of the leaves. In 

 most cases these incrustations are very soluble in water and are soon 

 washed off, but in some of the xerophytic plants, where lime is abundant, 

 the incrustations are somewhat permanent. This is notably the case with 

 certain saxifrages. 



Whether the function of excretion from leaves is of advantage to the 

 plant has not yet been proved, but it is thought that, in view of the 

 common occurrence, it is likely to be of some advantage. At all events, 

 where the excreted liquid matter is washed down to the soil, it is likely 

 to affect that soil to a certain extent. A study of soil fertility must, 

 therefore, include a consideration of leaf excreta. 



Leaf excretions bear an important relation to the effect of fungicides 

 such as Bordeaux. They react upon Bordeaux rendering some of the 

 copper soluble and thus making the Bordeatix more toxic, often so much 

 so as to kill the leaves in the neighborhood of the watei'pores, which 

 are situated at the margin of the leaf. 



But the water-pores are not the only means of excretion in leaves. 

 Excretion may occur through the epidermal cells of leaves having little 

 or no cutin. This can be proved by applying distilled water to the sur- 

 face. In nature, rain water extracts substances through the surface. 

 This is concerned also with the application of fungicides or insecticides. 

 what is insoluble in the spray liquid may become soluble through the 

 reaction of the liquid drawn from the cells of the leaf. 



The explanation of the process of excretion involves the process of 

 12 



