14 Plowman.— The Comparative Anatomy and 
degree. A form of aerenchyma is found in the medulla of several species 
of Scirpus, notably S. jluviatilis, S. atrovirens, and S. microcarpus, and 
in Fuirena and Stenophyllus . Elsewhere the medulla is usually more 
or less compact. Sclerosis is conspicuously developed in the nodal 
diaphragms of Dulichium and in the nodal regions of Scirpus robustus, 
Fuirena , and certain species of Eriophorum and Car ex. 
The sclerotic tissue of the cortical region presents a number of 
types of adaptation to mechanical purposes, as described by Schwendener 
(47). In some cases there is a dense hypodermal zone of sclerenchyma, 
while in other forms the mechanical tissue may be confined to numerous 
or few, large or small hypodermal ribs, or these may even give place 
entirely to a zone of assimilatory tissue which is not infrequently in 
the form of typical palisade cells. Following in part the suggestion 
of Rickli (40), we may divide the Cyperaceae into two classes, based 
upon the relative prominence of mechanical and assimilatory tissue in 
the cortex. Those forms which possess but little mechanical sclerenchyma 
in the cortex, but which show well-developed assimilatory tissue and 
numerous stomata, may be termed the Chlorocyperaceae. To this 
group naturally belong those species which are scapose in habit, or of 
which the leaves are much reduced in size or number. On the other 
hand, those forms in which the assimilatory mechanism gives place 
wholly or in large measure to sclerenchymatous elements may properly 
be called the Sclerocyperaceae. 
As typical of the first class we may consider the culm of Scirpus 
robustus , a cross-section of which is shown in Fig. n. It will be observed 
that the fibro-vascular bundles are collateral, not numerous, and scattered 
irregularly throughout the section. The medulla contains very large 
schizogenous air-spaces separated by plates of parenchyma only one 
cell thick. The medullary bundles lie at the intersections of these plates. 
It should be noted in passing that these large air-chambers are divided 
into compartments by occasional transverse partitions or * bulkheads.’ 
De Bary (12) has shown that these partitions are made up of dense tissue 
in species growing in dry situations, while in aquatic species the partitions 
are composed of thin-walled, stellate cells similar to those found in the 
medullary plates of certain Juncaceae. Frequently the fibro-vascular 
bundles anastomose in their course, in which case the transverse strands 
lie in these ‘ bulkheads.’ 
The epidermis is covered by a thin cuticle, which in some small 
amphibious species is distinctly papillate. Numerous somewhat depressed 
stomata open into the thick assimilatory zone, which is made up of 
two or three layers of typical palisade cells. The mechanical tissue 
of the stem is reduced to a few very small hypodermal ribs and a few 
sclerenchyma strands embedded in the inner margin of the palisade 
