Tissues in Certain Monocotyledons. 33 
decisive proof of its occurrence as the chief factor in the 
development of the bundle. 
Our final conclusions then as to the development of these 
tracheides are as follows : — 
(1) The tracheides are formed by longitudinal growth only, 
each tracheide arising from a single cell, which may grow to 
30—40 times its original length, but remains uninucleate 
throughout its whole development. 
(2) As the secondary tracheides are formed in a region which 
has ceased to grow in length, their development is entirely 
by sliding-growth , and consequently the number of initial 
desmogen-cells from which they arise is very small. In the 
Yucca investigated, for example, only a single cell in each tier 
of the desmogen-strand can become a tracheide. 
There can be no doubt that the development of the 
tracheides in the primary bundles is similar, but as the latter 
are formed in a region which is still lengthening as a whole, a 
proportionately smaller amount of sliding-growth is involved. 
The process in the case of the secondary bundles is a highly 
remarkable one and vividly recalls the invasion of a tissue by 
the hyphae of a luxuriant parasitic fungus. The initial cells 
from which the tracheides develope, might be compared to 
the spores of the fungus. We tried in vain to determine the 
position in the desmogen-strand of the initial cells for the 
tracheides. They certainly do not form a continuous longi- 
tudinal row. We believe that they may occur in any part of 
the strand, with the obvious exception of that group which is 
destined to form the phloem 1 . 
As our investigation has completely confirmed the views of 
Krabbe and Roseler, and the later view of Strasburger, it 
may be thought our duty to offer some explanation of the 
contradictory result obtained by Kny and Mdlle Loven, a 
result which one of us formerly accepted. From our own 
experience we think the chief sources of error are the 
following : The course of the tracheide as it passes between 
the overlapping and bevelled ends of the other elements of 
1 Cf. Krabbe, 1 . c. p. 64. 
D 
