THE STEM BB 
Structure of In order to study the minute structure of a 
Stemof stem, very thin — | 
Sunflower : 
ander sections must be 
Microscope. prepared and ex- 
amined under the microscope 
(Appendix A.). It will then 
be seen that the cells of  — : “ 
the epidermis of the Sun- py¢, 23,— Perrone or Duzap- 
flower are almost square, have NETTLE, TRANSVERSE SECTION. 
; (Diagrammatic. ) 
cuticularised outer walls, and ¢, central bundle; ¢, lateral 
no cell - contents. They are bundles; w, wing bundles. 
entirely protective. The cells | 
of the cortex next the epidermis are thin-walled, with large 
cavities, and vary very much in size and shape; where the 
walls of two or three cells meet they are thickened. The 
cells of the innermost layers of the cortex have not this 
thickening, and are parenchymatous. 
The fibro-vascular tissue consists of bast, cambium, and 
wood; the bast and wood are parallel to each other, and the | 
arrangement is known as collateral (Fig. 18). The boundary 
of the stele is clearly marked out by the bundle-sheath, or 
_entodermis (Plate III., end), a layer of cells rather longer than 
they are broad, and containing starch. Within this comes a 
layer of fibres, which may be distinguished from the under- 
lying cells by the rather thicker walls. These fibres are 
developed from the layer which immediately surrounds the 
central cylinder of vascular tissue, and is therefore called the 
pericycle (Gk. peri, round) ; the fibres are known as the pericycle 
fibres. Within this comes the true fibro-vascular tissue, con- 
sisting of bast, cambium, and wood. The external layer of 
the bast, that next the pericycle, consists of prosenchymatous 
tissue, known as bast fibres. This is a supporting tissue, by 
means of which, partly, the plant maintains its upright position 
against strong winds, etc. These two sets of fibres, the peri- 
cycle and bast, which are very similar to each other, together 
offer strong resistance to mechanical forces. Even when most 
delicate they ave found to be very tough and difficult to tear ; 

