ANGIOSPERM^E 375 



The vascular bundles (Fig. 346) have upon the inner side a group of tracheary 

 tissue, composed mainly of spiral or reticulately marked vessels, which are often 

 of large size. With these are associated elongated parenchyma cells. The outer 

 part (phloem) is made up of similar elongated parenchyma, mingled with sieve- 

 tubes. 



Monocotyledons are. usually perennial plants, but in cooler regions 

 the aerial shoots are sent up each year from the underground stem, 

 which ,may be a rhizome (e.g. many Grasses, Iris Germanica, Smila- 

 cina, etc.), a scaly bulb (Liliuni, Erythronium, etc.), or a corm 

 (Gladiolus, Brodiaea). The aerial shoots are often of very brief 

 duration, as in Erythronium, Tulip, etc., and the green shoots live 

 only long enough to ripen the seeds and prepare the starch and 

 other substances which are stored up in the underground stem for 

 next season's growth. Where the growth of the aerial shoots is 

 interrupted by drought, as in many Calif ornian and Cape bulbous 

 plants, the bulbs are small, and the growth of the new shoots is 

 only in a small measure dependent upon the reserve-food stored up 

 in the bulb. 



Sometimes the growth of the aerial shoots is extraordinarily rapid. 

 Thus in some of the large species of Bamboo, the shoots attain a 

 height of thirty to forty metres, this whole growth being completed 

 within a few weeks' time, and a growth of nearly a metre has been 

 recorded in twenty -four hours. 



Secondary Thickening. Where the stems are perennial, as in 

 Yucca, Dracaena, and Pandanus, there may be an increase in diame- 

 ter, such as occurs in Gymnosperms, but it is caused in a different 

 way. There is not a ring of vascular bundles, with cambium, but 

 the section of the stem (Fig. 345) shows the typical monocotyledon- 

 ous structure, with numerous scattered bundles. In the outer cor- 

 tex, however, a zone of meristematic tissue is found, in which new 

 bundles are formed as well as new ground-tissue. In such forms 

 the growth rings are either very obscure or quite unrecognizable. 



In the arborescent Monocotyledons, like the Palms and Yuccas, 

 the leaves often persist for several years, and when they drop off, 

 they may leave a definite scar. Where, as in the Cocoanut and 

 Royal Palm (Oreodoxa), the base of the leaf forms a sheath about 

 the apex of the stem, these scars form clean rings surrounding the 

 trunk at regular intervals. 



Climbing Stems. Climbing stems are comparatively rare among 

 Monocotyledons. Various tropical Aracese (Philodendron, Pothos, 

 etc.), Smilax, some species of Asparagus, Dioscorea, Vanilla, are 

 exceptions to the rule. 



Branching. The branching of the stem is almost always mono- 

 podial, and the branches arise in the axils of the leaves. A dichot- 

 omy of the apex probably takes place in the peculiar " Dom-palm " 



