THE MONOCOTYLEDONS 



411 



lateral ones are weak and degenerate. The fibrovascular bundles 

 of the leaf in monocotyledons are ordinarily very numerous and 

 consequently enter the stem in large numbers at the nodes. The 

 large number of foliar bundles passing from the base of the mono- 

 cotyledonous leaf into the stem is correlated with a high degree 

 of assimilative efficiency which finds expression in a proportion 

 of seed production which has scarcely ever been reached in her- 

 baceous dicotyledons. In many of the cereals, for example, the 

 relative weight of the seed to 

 that of the whole plant very 

 frequently reaches over 30 per 

 cent. The high efficiency of the 

 group, both from the standpoint 

 of production of assimilates and 

 from that of the formation of 

 seeds, naturally puts it in a 

 unique position in supplying 

 important food plants. 



In many cases, particularly 

 in the grasses and sedges, cam- 

 bial activity, absent in the stem 

 and root, is often retained in 

 the basal or sheath region of the 

 leaf or sometimes in relation to the node in the stem. The capacity 

 which grasses manifest for erecting their stems after "lodging" is to 

 some extent the result of the presence of a persistent cambium in 

 the nodal region, either in the base of the leaf or in relation to the 

 stem itself. Fig. 285 illustrates such cambial activity in the case 

 of Avena barbata. It is permissible to view this cambial activity 

 as a persistence of an ancestral character, particularly as it is often 

 found to be present in monocotyledonous seedlings hi the lower 

 region of the epicotyl or primitive stem. 



The organization of the closed fibrovascular bundles of the 

 monocotyledons is in many cases collateral and, as the descriptive 

 term implies, exhibits no indication of cambial activity. The 

 collateral type of fibrovascular strand is characteristic of the leaf, 

 since that organ, here as elsewhere, is conservative in its structure. 



FIG. 285. Transverse section of a 

 bundle of Avena barbata, showing cambial 

 activity (after Chrysler). 



