194 PLANT-LIFE 



a number of adventitious roots produced from the 

 stem. A transverse section of the stem shows uniformly 

 scattered vascular bundles which develop no cambium, 

 hence annual cambium " rings," by which the age of a 

 plant can be determined, do not appear; in fact, many 

 monocotyledons die down annually, although their under- 

 ground parts may be perennial. The leaves are usually 

 elliptical, elongated, or linear in shape, and in the great 

 majority of cases their veins are parallel. The flowers 

 betray a marked predilection for the number three; 

 this means that very often their parts are arranged in 

 whorls of three. Thus the perianth may consist of two 

 whorls composed of three sepals and three petals, the 

 stamens may consist of one or two whorls of three, while 

 the pistil is usually formed from a whorl of three carpels 

 united, in which event the ovary is three-celled. Sepals 

 and petals may both be coloured and so much alike as 

 to be indistinguishable. 



Monocotyledons are less numerous than Dicotyledons, 

 and, generally speaking, they are simpler in structure; 

 yet we have to remember that within their ranks we 

 have plants, like the Orchids, which are very highly 

 specialized. However, taken as a whole, they may be 

 regarded as being more primitive than the more com- 

 plicated, varied and numerous dicotyledonous Angio- 

 sperms. 



It is among aquatic Monocytoledons that we find 

 the simplest forms, but it is difficult to determine 

 whether these are primitive; it is possible that their 

 simplicity is an adaptation to environment ; that it may 

 be due to reduction from a more elaborate development 

 sort of discarding of useless machinery. Of these 



