22 
Plowman . — The Comparative Anatomy and 
The amount of mechanical tissue is relatively very large, and the mesophyll 
is sometimes almost wholly lacking in the mature condition, owing to the 
development of large air-cavities in the body of the leaf. The parallel 
fibro-vascular bundles or * veins ’ are very numerous, there being as many as 
sixty in some species. The middle one is considerably larger than the 
average, and in the broader leaves the bundle at the top of each arm of the 
V is also larger, so that in this case there are three principal veins. 
The bundles are always collateral, with well-developed peridesm, which 
is produced from each pole into a strong hypodermal rib. The assimilatory 
zone is rarely differentiated into a true palisade layer. The upper epi- 
dermis is covered by a strong cuticle, and is frequently developed into 
short, sharp, sclerotic spines, especially along the margins of the leaf. Over 
the hypodermal ribs the epidermal cells are smaller and thinner-walled, 
as already seen in the stem, and as described by De Bary (12) and 
Spinner (50). The epidermis of the under surface is thinner-walled, and 
commonly bears more or less conspicuous cuticular ‘ pegs ’ or papillae, 
at least in all aquatic and limicolous species. The stomata are relatively 
few and usually somewhat depressed. In their essential structural features 
the leaves of the Cyperaceae show little, if any, more variation than do the 
roots. Eichler (15) and Trecul (52) have shown that the leaf of the Cype- 
raceae is basipetal in its growth, and that the fibro-vascular strands are 
projected inward to their insertion upon the cauline system. 
Both Guillaud (19) and Gwynne- Vaughan (20) have emphasized the 
great importance of the leaf in the Monocotyledons as a determining factor 
in the development of the central cylinder of the stem. The latter observer 
points out the fact that in the apical growth of the young stem the terminal 
cone is altogether insignificant in comparison with the latest-formed leaf, and 
the procambial strands of the young leaf are more numerous and larger 
than those of the cauline ring where first differentiated. 
We have already alluded to the fact, established by the investigations 
of Chrysler (9) and others, that in an early stage of its growth the stem of 
a monocotyledonous plant has its fibro-vascular elements disposed in a 
single circle of collateral strands, and that it is only at a later stage that 
medullary strands, collateral or amphivasal, make their appearance. That 
is to say, it is only after the introduction of a number of leaf-traces into the 
cauline system that we find the characteristic Monocotyledonous type and 
arrangement of bundles making their appearance. It is a well-known fact 
that the leaf-trace of the Monocotyledons is made up of a far greater 
number of bundles than is that of any other group, and, as Jeffrey (30) 
points out, it is probably the accommodation of this large number of 
incurrent bundles that accounts for the dense nodal complexes which are so 
common a feature of this group. These bundles do not end blindly 
in the medulla, according to the ancient doctrine of Desfontaines, but each 
