DEVELOPMENT OF PLANTS 367 



Class A. Monocotyledones 



129. General Characteristics. — This group of angiosperms con- 

 tains about 25,000 species that constitute a very natural alliance, 

 owing to the uniformity and simplicity of their structures. These 

 features may be due to the uniform conditions under which these 

 plants live. The majority of Monocotyledones are moisture- 

 loving plants and are therefore exposed to very constant condi- 

 tions, which would naturally result in less stimulation and conse- 

 quent variation than in the case of plants exposed to the varying 

 conditions of drier soils. Some, to be sure, have become adapted 

 to dry and even arid regions, owing to peculiar modifications of 

 their leaves or stems, as in the grasses, certain bulbous plants, etc. 



The leaves are smooth and simple, lance-shaped or linear in 

 outline, sessile and often attached to the stem by sheathing bases. 

 The veins do not end in free branches on the margins of the 

 leaves, and as a result of this closed venation the leaves are usu- 

 ally entire and destitute of teeth or lobed margins. In many 

 instances the principal veins are quite parallel, but whatever the 

 arrangement, the prominent veins are connected by very minute 

 veinlets that form an inconspicuous network or reticulation 

 throughout all parts of the leaf (Fig. 277). The stems are com- 

 posed largely of parenchyma, through which are scattered numer- 

 ous veins or vascular bundles derived entirely from the leaves 

 (Fig. 278). These bundles do not develop a cambium (Fig. 60) 

 and consequently the stem does not increase materially in diame- 

 ter, and usually it is columnar in appearance. In many instances 

 the stems are reduced in size and subterranean, the bulb and 

 rhizome being common forms of stems which send up annually 

 short-lived aerial branches. As a rule the stems do not branch, 

 owing to the failure of the buds in the axils of the leaves to 

 develop. 



The flowers are also of rather simple and uniform structure, 

 in the simplest cases being imperfect and consisting of one or 

 more naked stamens or of one or several pistils. In the higher 

 types spiral flowers with perianth and both kinds of sporophylls 

 appear, and these give place to forms in which the organs are 



