SECT. II 



PHANEROGAM IA 



463 



Fio. 898.— Diagram of a 

 typical Monocotyle- 

 donous flower. 



The primary root dies prematurely and is replaced by adven- 

 titious roots, which usually live but a short time, and are in turn 



superseded by others developing successively higher and higher on 



the stem. The roots are generally unbranched, 



and exhibit secondary growth in thickness only ® 



in the few cases when a cambium is present in 



the stem. 



The stem of most Monocotyledons is simple ; 



when branching does occur, it rarely results in 



the formation of a profusely-branched crown (with 



respect to the disposition and structure of the 



vascular bundles of the stem, see p. 102 ; for 



occurrence and description of secondary growth, 



p. 138). The leaves are always devoid of stipules, 



and, in the majority of cases, alternate, arranged 



in two or three ranks. They generally have a 



well-developed sheathing leaf-base, are without stalks, and are lineal 



or elliptical in shape and parallel-nerved, although leaves otherwise 



constructed not unfrequently occur (Fig. 399). 



The structure of Monocotyledonous flowers may be traced back, 



in almost every case, to the trimerous pentacyclic type (Fig. 398). 



It may accordingly be inferred that the flower of the ancestral form 



was actinomorphic, and composed of five alternating trimerous whorls, 



each whorl consisting of similar members. 



This type has been retained unchanged in many Monocotyledons ; 

 in others, modifications have occurred in the course 

 of their phylogenetic development, resulting some- 

 times in a transition from an actinomorphic to a 

 zygomorphic or asymmetrical structure, sometimes 

 in a reduction in the number of members in the 

 whorls, less frequently in an increase. The more 

 important of these deviations from the usual type 

 will be noticed in detail in the descriptions of the 

 single families. 



The perianth is not usually differentiated into a 

 calyx and corolla ; it is small and inconspicuous or 

 large and highly coloured according to the mode 

 of pollination, whether effected by the wind or 

 insects. In a few cases of entomophilous pollina- 

 tion the perianth remains insignificant, and other 

 parts of the plant assume the function of an 



attractive apparatus. 



Pig. 399.— Polygonatum 

 mvMiflorum, leaf with 

 parallel venation. 

 (Nat. size.) 



The Monocotyledons are divided into the following orders : Liliiflorae, Enan- 

 tioblastae, Spadiciflorae, Glumiflorae, Selobiae, Scitamineae, Gynandrae. These 

 orders do not constitute a continuous series, beginning with the most primitive 

 forms and successively ascending to those more highly developed, but represent 



