HISTOLOGY 



21 



imbedded in it, and it is covered externally by a protective layer 

 of cells, the epidermis. The stem is usually solid in all grasses in 

 the young stage, but as it matures the internodes become hollow 

 in many grasses and they remain solid in a few. In the internodes 

 the fibro-vascular bundles run longitudinally and are parallel, but 

 in the nodes they run in all directions and form a net work from 

 which emerge a few bundles to enter the leaves. So far as the 

 broad general features are concerned, the stems of many grasses are 

 more or less similar in structure. However, when we take into con- 

 sideration the arrangement of bundles,the development and arrange- 

 ment of sclerench) ma, every species of grass has its own special 

 characteristics. And these are so striking and constant that it 

 may be possible to identify the species from these characters alone. 

 We may take as a type the stem of Rottboellia exaltata. This 

 stem is somewhat semi-circular in transverse section and it is 

 almost straight and flat in the front (the side towards the axillary 

 bud). The peripheral portion of the stem becomes somewhat 

 rigid and thick due to the aggregation of vascular bundles, some 

 small and others large. The outermost series of bundles consisting 

 of small and larger bundles are in contact with the layers of the 

 cells lying just beneath the epidermis and these cells are also 

 thick-walled. A few are away from these being separated by three 

 or four layers of cells from the peripheral bundles. In all these 

 vascular bundles the bundle-sheath is very strongly developed all 

 round and is very much developed especially at the sides. It is 

 this great development of sclerenchyma that makes the outer 

 portion of the cortex hard. Within the ground tissue are found a 

 number of vascular bundles scattered more or less uniformly. 

 These bundles have no continuous bundle-sheaths but have instead 

 groups of fibres at the sides and in front of the phloem. The 

 cavities near the annular vessels are somewhat larger and conspi- 

 cuous in these bundles. 



Kin. 25. — Transverse section of the stem of Pennisetum cenchroideS, x 20 



The epidermal cells are all thickened very much and the outer 

 layer is cutinized and impregnated with silica. This is the case in 

 the epidermis of the stems and leaves of most- grasses. (See fig. 24.) 



