CII. I] 



The Morphology of Grasses 



growth. ' If however the shoots pierce through the sheaths 

 close to where they arise and grow out horizontally (the extra- 

 vaginal mode of growth), the stems and foliage become more or less 

 scattered and intermingled with other plants. Thus the appearance, 

 habit, and agricultural use of a particular species depend largely 

 upon whether the shoots are mainly intra- vaginal, or extra-vaginal 

 in their mode of growth. In the Oat, as in all annual grasses, 



Fig. 3 



Fig. 2. A. Portion of culm of the cultivated Oat Avena sativa. B. A longi- 

 tudinal section of the same showing the structure of a node. The swollen 

 appearance of the node is due to the enlargement of the base of the leaf- 

 sheath (sh). The actual stem-node or joint (n) is not enlarged. This is 

 the case in most grasses. x 2. 



Fig. 3. Diagrammatic transverse section through internode of Wheat culm, 

 showing the position of the strongly developed ring of sclerenchyma (Scl.) 

 (thick walled tissue) just beneath the epidermis. It is this band of scleren- 

 chyma which largely contributes to the firmness and strength of grass culms. 

 Note also the ring of principal fibro-vascular bundles (Fj), with secondary 

 bundles between ( V 2 ). x 15. See also Fig. 4. 



there is only one kind of stem — the flowering culm — but many of 

 the perennial species have also the second type of stem (extra- 

 vaginal) which grows out horizontally either on or below the 

 surface of the soil. Couch-grass affords an excellent example of the 

 formation of underground stems or rhizomes, while Rough-stalked 

 Meadow-grass (Poa trivialis, L.) and Fiorin (Agrostis alba, L. var. 

 slolonifera) and others possess thin surface-creeping offshoots — or 



1—2 



