214 PRACTICAL BOTANY. 



sheath correspond in position to the channels of the 

 next higher internode ; since the teeth are continuous 

 downwards with the ridges of the lower internode, it 

 follows that the ridges of the lower internode alternate 

 in position with those of the internode next above it. 



Strip off carefully one leaf-sheath, and it may then 

 be clearly seen that the ridges of the upper internode 

 alternate with those of the internode next below it. 



II. Cut transverse sections from a mature internode 

 of an upright aerial stem : mount some in glycerine, 

 others in Schulze's solution, and examine first with a 

 low power : observe 



i. The sinuous outline of the section, the projections 

 corresponding to the ridges observed externally with 

 the naked eye, and the indentations to the channels 

 intervening between them. 



ii. The section is limited at the periphery by an ill- 

 defined layer of epidermis, which, together with sub- 

 jacent tissues, forms a band of thick-walled 

 tissue of very variable breadth ; thus the band is broad 

 at the most convex parts of the ridges, and at the most 

 depressed parts of the channels, while it is reduced on 

 the sloping sides of the ridges to the single layer of 

 thick-walled epidermal cells. 



iii. Beneath this is a broad band of Cortical tissue, 

 in which may be recognised 



a. Groups of Chlorophyll-parenchyma of oval 

 outline; one of these lies opposite to each of the 

 ridges, and extends to points close below the surface 

 of the sloping sides. 



6. Parenchyma composed of rounded cells with little 

 or no chlorophyll. 



