254 



PLANT STRUCTURES 



FIG. 237. A group of orchids ( Cattleya), showing the very zygomorphic flowers, the 

 lip being well shown in the flower to the left (lowest petal). CALDWELL. 



DICOTYLEDONS 



137. Introductory. Dicotyledons form the greatest group 

 of plants in rank and in numbers, being the most highly 

 organized, and containing about eighty thousand species. 

 They represent the dominant and successful vegetation in 

 all regions, and are especially in the preponderance in tem- 

 perate regions. They are herbs, shrubs, and trees, of every 

 variety of size and habit, and the rich display of leaf forms 

 is notably conspicuous. 



Two great groups of Dicotyledons are recognized, the 

 Arcliichlamydece and the Sympetalce. In the former there 

 is either no perianth or its parts are separate (polypeta- 

 lous) ; in the latter the corolla is sympetalous. The Archi- 

 chlamydeae are the simpler forms, beginning in as simple a 

 fashion as do the Monocotyledons ; while the Sympetalae 



