1912] THOMPSON—RAY TRACHEIDS 335 
cells in the summer wood of the previous year (figs. 7, 8, 11). These 
groups of parenchyma cells at the boundary of the annual ring are 
characteristic features of many species of Abies. In the spring 
wood they disappear, to be revived again in the following summer 
wood. The result is the appearance of single cells or small groups 
of cells scattered along the margins of the rays, each group at the 
end of an annual ring. Between the groups there is usually no 
trace of cells, but here and there peculiar “ ghostlike” appearances 
resembling cells may be seen. One of them is shown in fig. 6, 
which is a photograph of a section of A. homolepis. They are 
shadowy, structureless affairs, staining best in hematoxylon and 
giving no response to the phloroglucin test for lignin. Occasion- 
ally they form an almost continuous cell-like series from one group 
of parenchyma cells to the next, but usually they are rather isolated. 
That these structures are not merely thin sections of the sides of 
actual living cells is shown by their staining qualities, by their lack 
of structure, and by a study of a series of sections. The only 
explanation for their presence is that they are degenerated cells. 
In fact, it is possible to observe, especially in the cambial region, 
all stages in degeneration from such a conspicuous object as that 
seen in fig. 6, to ones which are barely discernible even with the 
strongest staining. The scattered condition is,obviously due to 
the complete disappearance of some members of the series. 
Now when ray tracheids occur, they identify themselves defi- 
nitely with these series of degenerating cells. They may often be 
observed conterminous with one of the “‘ghosts.’’ The inference 
seems irresistible that these “ghosts” were originally complete 
rows of ray tracheids which have degenerated in most species, but 
may survive sporadically in a few, and may be recalled traumatically 
in others. 
The groups of parenchyma cells at the end of the annual ring 
are likewise definitely identified with these degenerating cells, 
whether ray tracheids are present or not. It seems probable that 
at the end of the year’s growth, when there is a great demand for 
storage in preparation for the approaching winter, the cells which 
would otherwise degenerate are given a new lease of life and trans- 
formed into parenchyma cells which are capable of storage. That 
