GENERAL INFORMATION 113 



as to warrant the assumption that they have had a 

 common ancestor. Characters taken as generic and 

 specific are analogous to theories or working hypoth- 

 eses. We use them as far as they work ; if they do not 

 work, we discard them for other characters. Botany 

 is a science of living things, and its problems can 

 never be settled once for all. It is this enduring 

 interest that makes it so fascinating. 



POSITION OF GLASSES IN THE PLANT KINGDOM 



Flowering plants (excluding the pines and their 

 allies) fall into two rather distinct groups, (1) mono- 

 cotyledons, characterized by an embryo having a 

 single cotyledon and by stems having woody fibers 

 not in layers but distributed through them (as seen 

 in the cornstalk) and not increasing in thickness by 

 age, and (2) dicotyledons, with an embryo having 

 two cotyledons and stems with their woody fibers 

 forming a zone between pith and bark and increasing 

 in thickness by annual layers. Cotyledons are the 

 seed leaves. Anyone will have observed that sprout- 

 ing corn, rye, and other grasses send up a single leaf 

 first, while squash, radishes, lettuce, and morning- 

 glories, for example, have a pair of opposite seed 

 leaves. Grasses belong in the first class, with sedges, 

 rushes, lilies, and the like. They form a highly 

 specialized family with a greater number of species 

 than any other except the orchids and the com- 

 posites (asters, dandelions, thistles, and their kinds). 



