68 PLANT PHYSIOLOGY 



gate saprophyte; if it can live also by some other means, it is a 

 facultative saprophyte. Many of the higher plants can use sugar 

 in solution and, in this sense, they are then facultative saprophytes. 

 In a similar way a plant which must live parasitically is an obligate 

 parasite; while, if this is only one method of obtaining a living, 

 it is only a facultative parasite. Thus the same plant may be a 

 facultative parasite and saprophyte at different times in its life. 



Sporophyte and Gametophyte. — In the evolution of the plant 

 kingdom, the appearance and development of the sporophyte 

 generation, accompanied by the corresponding decrease in the 

 gametophyte, are connected with important changes in the methods 

 of securing food. In the Thallophytes, where a sporophyte gener- 

 ation can hardly be said to exist, the gametophyte is autotrophic 

 in the Algae and parasitic or saprophytic in the Fungi. In the 

 Bryophytes, the well-organized sporophyte is for the most part 

 parasitic upon the gametophyte; it makes little or no food and 

 depends upon the chlorophyll of the gametophyte for its suste- 

 nance. In a few of the mosses, the stalk of the sporophyte (seta) 

 contains some chlorophyll, indicating that it is partly autotrophic ; 

 but, with the exception of a few Algae, not until the Pteridophytes 

 are reached, are entirely independent sporophytes produced, the 

 gametophyte (prothallium) at the same time still retaining its 

 independence. The gametophyte of the higher Pteridophytes, 

 e. g., Lycopodium, has almost completely lost, however, its auto- 

 trophic nature. The chlorophyll-bearing tissue of the gametophyte 

 gets less and less as the chlorophyllous tissue of the sporophyte 

 increases. It thus becomes more and more dependent, and finally 

 finishes in the Spermatophytes as a complete parasite. 



The young seedling begins life as a saprophyte, using the food 

 stored in the seed by the previous generation. As the plumule 

 unfolds exposing the leaves, an increasingly larger proportion of 

 the food is made by the plant, which thus becomes more and more 

 independent as the stored food decreases. Hence the one indi- 

 vidual within the short space of ten days during germination goes 

 through all the stages from obligate, complete saprophytism to 

 complete autotrophism. 



Injury Due to Parasites. — Plant diseases are generally associated 

 with physiological disorders. One type of plant disease is produced 

 by an improper physical environment; the plant lacks light, the 

 proper mineral salts, or some other essential to normal growth. 



