104 ANATOMY OF THE GVMNOSPERMS 



of the primitive character in the face of conditions which involve 

 a change. Following these are two species with simple pits, one 

 with transitional features, five with simple pits, one with bor- 

 dered pits, one with the transitional form, and the remaining six 

 species with simple pits only. It will therefore be seen that these 

 changes occur in waves, and that within the limits of forty-one 

 species there are three complete and six incomplete recurrent 

 phases. If we were arguing from purely theoretical grounds, all 

 of these species should be arranged in such order as to show (i) 

 bordered pits, (2) transitional forms, and (3) wholly simple pits, 

 and we should thereby gain a perfect developmental sequence. 

 But such a position would not be justified by other evidence of 

 an equally if not more weighty character, and it is our object 

 to interpret the facts as they are found. It has already been 

 shown that the occurrence of simple pits in the pines is conso- 

 nant with a higher type of development, and that the change is 

 not only accompanied by sporadic reversions or survivals, as one 

 may choose to regard them, but that the change as a whole is 

 a process of reduction. From this point of view, then, we must 

 regard the occurrence of bordered pits in P. clausa, P. rigida, and 

 P. pungens as pure survivals of a more primitive structure, --a 

 feature which is less perfectly expressed in such transitional forms 

 as P. koraiensis or P. mops. But a mere mingling of the two 

 kinds of pits in the same species is not the only evidence in this 

 direction. The mingling of simple and bordered pits does not 

 occur indiscriminately, but in accordance with a well-defined law 

 to the effect that the former are characteristic of the spring wood 

 throughout its entire extent, while the latter occur, if at all, only 

 in the summer wood, where they might be expected, since the 

 arrested development which might be complete in the case of 

 relatively thin-walled cells could be readily overcome in part in 

 walls of greater secondary growth. This in no way conflicts with 

 the observed fact that in the majority of cases the usual course 

 of development is such that the bordered pits of the spring wood 

 very commonly become reduced to simple pits in the summer 

 wood, in accordance with De Bary's law, as already stated in 



