1901 | ANATOMY OF THE OSMUNDACEAE 405 
to the long axis of the stem, and there is a gradual transition to 
the tangential position. More than this, the laterally placed 
protophloem cells of the leaf traces can be directly traced into 
the ‘‘ quergestreckete Zellen” of the stem. There seems little 
doubt, therefore, as to their nature. 
To summarize observations : (1) The “ quergestreckte Zel- 
len” are sieve tubes, as has been demonstrated above ; (2) they 
become differentiated at the same time as the typical proto- 
phloem, and (3) occupy the same relative position; (4) they 
resemble the protophloem cells in form ; (5) their orientation is 
not uniform; (6) they pass imperceptibly into the longitudi- 
nally orientated protophloem cells of the leaf traces. Hence 
there seems no reason to regard them as anything else than pro- 
tophloem, 
CONCLUSIONS. 
The question now remains, how to interpret the vascular sys- . 
tem of the Osmundaceae. To do this more intelligibly, it will 
be well to recapitulate the main fibrovascular theories. We shall 
begin with that of Sachs and DeBary. 
These botanists regarded the bundle as the unit, and the vas- 
cular system as a more or less simple complex of bundles 
embedded in ground or fundamental tissue. Developmental 
Studies have shown that this theory is inadequate, for the unit is 
wrong, 
The hypothesis which at present obtains was supplied by Van 
Tieghem and Strasburger. In this conception” the stele is 
the unit. The primitive form of stele, the monostele, such as 
occurs for example in most roots and in the stems of lycopods, is 
a solid central strand of xylem, surrounded by a sheath of phloem, 
and marked off from the cortex by the differentiated internal 
Cortical layer, the endodermis. Of this there are many modifi- 
Cations, of which mention is made of the most important. By 
the repeated bifurcation of the monostele, the polystelic type is 
VAN TIEGHEM: Traité de Botanique 673, 765- 
*® VAN TIEGHEM: Sur la polystélie. Ann. Sci. Nat. Bot. VII. 3: 275. 
*®VaN TieGHEM: Eléments de Botanique 1:84, 179- 
