STEM AND INFLORESCENCE 9 
obtaining, with economy of material, the maximum of strength and 
resistance to lateral strains, inasmuch as the cylinder formed by 
the tissues of the culm has its wall radially graduated in strength 
—vital tissue consisting of soft parenchyma and fibrovascular 
bundles innermost ; outside of this a cylinder of sclerenchyma, and 
at the periphery the silicified cells of the epidermis. The culms of 
some bamboos have their mechanical cells and epidermis so hard 
as to resist the blow of a hatchet. The closely investing leaf- 
sheaths give a great amount of support to the stem which would 
otherwise, in many cases, be unable to support its own weight. 
The immense saving of tissue, re- 
presented by the large central 
cavity of the culms, enables them 
to grow with great rapidity ; young 
bamboos have been observed to 
elongate 2-3 ft. in a day. A few 
grasses are exceptional in having 
the stems solid, eg. the genera 
Glyceria and Saccharum. If we 
examine a transverse section of the 
stem of Saccharum officinarum, 
the Sugar-cane, we find that the 
fibrovascular bundles are most 
numerous near the circumference, 
the central part consisting mostly 
of parenchyma, among which the 
bundles are scattered sparsely. 
The flowers of grasses are en- 
veloped in smail scales or bracts 
(glumes) which are imbricated or 
overlapped so as to form little 
spikes or clusters (spikelets). Thus 
in fig. 14 there are thirteen spike- 
lets represented; in fig. 26 there 
are ten spikelets. These glumes 
are the distinguishing feature of the 
Glumace@, a well-marked division 
of the Monocotyledous, and com- 
prising, besides the grasses, a few 
other orders, notably the Cyperacee, 
sedges. The upper part of the stem 
Fic. 5.—Transverse section of culm 
of Phragmites communis (segment of 
an internode, enlarged): C large cavity 
of culm; o its centre; the innermost 
(light-coloured) tissue is parenchyma in 
which are embedded the isolated fibro- 
vascular bundles, oval shaped, showing 
cavities (two wood-vessels), and exter- 
nally bast; the peripheral part of the 
section (dark-coloured) is sclerenchyma, 
s (mechanical cells), in which are em- 
bedded smaller or imperfect bundles, 
forming the axis of the inflores- 
cence is called the rachis ; it is nearly always simple, z.e. undivided ; 
but when two or more axes have a common insertion at the top of the 
stem, the rachis is termed compound (fig. 30). ‘The simplest form 
of inflorescence in grasses is the spike, each spikelet being sessile 
(ze, without a stalk or pedicel) on the rachis, and usually seated in 
notches or excavations (fig. 39) in two opposite rows (distichously) 
and alternately. More commonly the spikelets are borne on 
slender hairlike branches of the rachis, which have generally an 
alternate half-whorled insertion, and these branches are often 
