178 ANATOMY OF THE GYMNOSPERMS 



periods of development and under special conditions of environ- 

 ment, when they become recognizable without the aid of the 

 microscope by reason of their characteristic fruiting structures. 

 It is further true that in these two groups of plants there is 

 a more or less variable relation toward the source of food supply. 

 This is expressed by the classification long since adopted by 

 De Bary (83), and now generally used with slight modifica- 

 tions, who recognized : 



1. True or obligate saprophytes: those which obtain their food supplies 

 from the products of organic decay under all circumstances. 



2. Partial or facitltath'e saprophytes : those which usually complete the 

 life cycle as true saprophytes, but which, under special circumstances, may 

 more or less completely but temporarily become parasites. 



3. True or obligate parasites : plants which invariably derive their nutri- 

 tion directly from the nutritive materials of living organisms. 



4. Partial or facultative parasites: those which, under special circum- 

 stances, may become saprophytes, though ordinarily completing the life 

 cycle as true parasites. 



Among the very large number of parasites and saprophytes 

 which attack timber, either living or dead, it will be found that 

 within certain limits there is a more or less well-defined relation 

 to the organism affected, whereby it is characterized by the 

 growth of special forms. The number of species peculiar to a 

 given tree will be found to vary somewhat widely, and this will 

 in turn be influenced within the limits of a particular species of 

 tree by conditions of environment. Thus von Schrenk (68, 49) 

 shows that the wood of Catalpa speciosa is injuriously affected 

 by only two fungi, and the same is likewise true of the red cedar 

 (Juniperus virginiana); but Dudley (14) points to the fact that 

 no less than eighteen species of fungi infest the wood of the 

 hemlock (Tsuga canadensis), of which nine are Polypori and six 

 Agarics. Again, it is a well-known fact that other fungi, such as 

 dry rot (Merulius lachrymans), are not selective in any particu- 

 lar sense, but by reason of their very cosmopolitan habits they 

 grow within any wood, provided the external conditions of warmth 

 and moisture are favorable. It by no means follows from the 



