Scott and Hill.-—Structure of 1 soetes Hystrix. 421 
divisions around the periphery of the stele were well marked, 
at a level above the commencement of lignlfication of the 
primary tracheides. These divisions take place in cells 
immediately outside those which differentiate into the wood 
of the stele. No demarcation between pericycle and cortex 
could be detected. 
The question of the existence of phloem in the stem of 
Isoetes has been left in a very unsatisfactory condition, so 
much so that the latest writer on the genus, Wilson R. Smith 
(1900, p. 227), proposes to drop the application of the word 
phloem altogether ‘ until its justification shall be established 
on physiological grounds.’ This seems rather a remote con¬ 
tingency in the case of a genus like Isoetes , and it is well that 
the anatomy does not leave us so entirely in the dark. Mr. 
Wilson Smith treats the whole of the so-called prismatic zone, 
i. e. the secondary tissue lying on the inner side of the cambium, 
and thus immediately surrounding the primary cylinder, as 
one tissue, and argues against its being regarded as phloem, on 
the ground that some of its elements may become converted 
into tracheides. As a matter of fact the ‘ prismatic zone ’ 
within the cambium is sharply differentiated into three kinds 
of tissue—secondary parenchyma, secondary wood, and the true 
phloem. As was first detected by Hegelmaier (1874, p. 5 00 )j 
this zone consists of alternating, concentric bands of starch- 
containing tissue, and of cells without obvious contents. The 
existence of this differentiation can always be detected, though 
more obvious In the older stems. The starch-containing cells 
retain their nuclei, while the phloem-cells soon lose theirs; 
In young phloem-cells, granules which probably represent the 
disorganizing nuclei are often present. The presence of an 
enucleate zone is easily recognized, even apart from other 
characters, in stained sections. It is usually first differentiated 
immediately within the cambium, and separated from the 
primary wood by a layer or two of parenchyma (see Fig. 5). 
The phloem-elements have an extremely characteristic struc¬ 
ture of their cell-walls, which comes out conspicuously in 
sections deeply stained with haematoxylin. Their walls are 
