SPERMATOPHYTES 239 



flowers of monocotyledons have their members in sets of three; while 

 the flowers of dicotyledons have their members in sets of five or four. 



These characters are by no means of equal value, the character of the 

 embryo being the only one without serious exception. There are mono- 

 cotyledons with vascular cylinders, with open venation, and with flowers 

 not in threes ; and there are dicotyledons with scattered vascular bundles, 

 with closed venation, and with flowers in threes. It is not so much a 

 single character, therefore, that distinguishes a monocotyledon from a 

 dicotyledon, as a combination of characters. 



There are recognized also two great divisions of dicotyledons, the 

 Archichlamydeae and the Sympetalae, and the conspicuous character 

 which distinguishes them is implied in their names. The Archi- 

 chlamydeae have either no petals or petals entirely separate from one 

 another (free), and this is recognized as the primitive condition of the 

 perianth (chlamys); while in the Sympetalae the petals develop so as 

 to form tuljes of various kinds. This distinction is superficial and 

 breaks down in certain cases, but it holds generally and is convenient. 



These three great groups of angiosperms are related to one another 

 as follows: the Archichlamydeae are recognized as including the most 

 primitive angiosperms; from the more primitive Archichlamydeae 

 the monocotyledons probably have arisen as a special branch; while 

 from the more ad-\ranced Archichlamydeae the Sympetalae have arisen 

 and are clearly the highest group of angiosperms. It will be necessary 

 to keep in mind these three groups and their relationships in order to 

 understand the following discussion. 



Stem 



A description of the general structure of a vascular stem has been 

 deferred to angiosperms, whose stem may be taken as an illustration 

 of the general features of all vascular stems. 



Elongation. — The tip of the stem is the growing point, consisting 

 of a .group of very actively dividing (meristematic) cells. Among 

 pteridophytes this group of meristematic cells is usually represented 

 by a single apical cell. All the tissues of the stem are derived from the 

 cells of the growing point, whose activity very soon results in the 

 appearance of three more or less distinct generative regions : (i) der- 

 matogen, which later forms the epidermis; (2) periblem, which later 

 forms the cortex; and (3) plerome, a central cylinder which forms the 

 stele, in which the vascular elements appear (fig. 538). 



