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Notice of Book . 
dinal sliding growth, if branching of the cells be left out of considera- 
tion. If the total number of mature tracheides seen in a transverse sec- 
tion be called x, the number of sub-cambial cells from which they are 
oc 
derived will be - 5 where m is the ratio of the length of the mature 
m 
tracheide to that of the sub-cambial cell. In Dracaena Draco the 
average value of x is thirty-eight, and that of m is also thirty-eight. 
Hence the number of sub-cambial cells at one level, which become 
tracheides, will be one, i.e. the whole system of tracheides in each 
bundle arises from a single vertical row of sub-cambial cells. Here 
then, according to Dr. Krabbe, we have a most striking case of the 
formation of a system of tissue from a simple row of cells, without 
the aid of any further cell-division. The tracheides form by far the 
most important constituent of the bundle, and the author estimates 
that fourteen-fifteenths of the whole sectional area of the bundle are 
occupied by tissues developed by sliding growth. Thus, on this 
hypothesis the central phloem must gradually become enveloped by 
the growing tracheides, as by a web of hyphae. It is unnecessary to 
enter into a more detailed consideration of this case or of the similar 
phenomenon alleged to occur in Aloe and Yucca. It will be well 
known to many readers, that a totally different explanation of the de- 
velopment of the so-called tracheides of these plants has been given by 
Professor Kny 1 , in a work published almost simultaneously with that of 
Dr. Krabbe. Professor Kny finds that the elements in question are not 
tracheides at all, but short vessels, arising from the fusions of a series 
of sub-cambial cells. If this be the case, their formation does not 
necessarily involve sliding growth, though Professor Kny allows that 
this may occur to a small extent at the ends of the elements. 
So direct a contradiction in the results obtained by two competent 
observers is certainly remarkable, and further investigation is urgently 
required. The writer of this article has himself made some obser- 
vations on this subject, but they are not yet sufficiently complete for 
publication. The very pointed ends and occasional branching of the 
tracheides indicate that a certain amount of sliding growth must take 
place, but this by no means excludes the possible occurrence of 
cell-fusions. 
1 Beitrag zur Entwickelungsgeschichte der ‘ Tracheiden.’ Berichte der deutschen 
bot. Gesellschaft. Bd. IV, S. 267, 1886. 
