THE MONOCOTYLEDONES 2069 



The axis of the flower stands in the axil of the outer palea and bears a 

 scaly bracteole called the inner palea, opposite to the outer palea. The 

 flower thus lies between the two paleae. There are no sepals or petals. 

 Opposite the inner palea on the outer side of the flower are two small scales 

 termed the lodicules. Many consider these to represent the remains of 

 perianth segments which otherwise are entirely suppressed. An alternative 

 view is that they represent a second bifurcated bracteole. The inner palea 

 itself is sometimes bifurcate, which supports this hypothesis. The lodicules 

 are biologically important because they are hygroscopic, and their move- 

 ments separate the paleae and open the flower at anthesis. 



The androecium consists typically of three hypogynous stamens 

 corresponding to the outer whorl of the normal monocotyledonous flower. 

 Occasionally, e.g., Bambusa, there is an inner, alternating whorl, also cf 

 three stamens. Each stamen has a long filament and bears a versatile 

 anther which dehisces extrorsely. 



The gynoecium is monocarpellary and usually bears two feathery 

 stigmas. It should be noted that these stigmas arise from the carpellary 

 wall and not from the apex and the presence of two is not necessarily 

 an indication of a bicarpellary ovary. The ovary is unilocular and superior, 

 and contains a single, erect, anatropous ovule. 



The fruit is a caryopsis, i.e., an akene in which the pericarp is adherent 

 to the seed. 



The seed is endospermic and the embryo lies to one side. It is straight, 

 with a shieldlike appendage, called the scutellum, on the side adjacent 

 to the endosperm (see p. 1592). Radicle and plumule are well developed, 

 but the radicle is very short-lived and is soon replaced by adventitious roots. 



The family is divided into over 450 genera including, at a conservative 

 estimate, 4,500 species, which are to be found in all regions of the globe. 

 In temperate regions they form the most important constituent of the 

 vegetation covering such vast areas as the prairies and steppes. 



Anatomically the grasses show certain peculiarities. The long internodes 

 are usually hollow, due to the failure of the pith to develop, though a 

 transverse diaphragm containing a complex of leaf trace bundles is usually 

 found at the nodes. In many species the upper surface of the leaf is longitu- 

 dinally ridged. The outer layers of these ridges are composed of scleren- 

 chyma, while the stomata are restricted to the grooves between. In many 

 xerophytic grasses the assimilatory tissue is restricted to the sides of the 

 grooves, and by the shrinkage of a row of large parenchyma cells lying at 

 the base of each groove the whole leaf may be rolled up longitudinally when 

 the air is dry. 



Even in grasses not growing under xerophytic conditions the stomata 

 are developed in lines and there are large subsidiary cells. The venation ot 

 the leaves is parallel and the vascular bundle is often surrounded by a 

 sclerotic sheath. The young leaves in the bud are usually convolute but in 

 some instances are conduplicate and more or less elliptical or compressed 

 in section. 



