244 



MORPHOLOGY 



with the insertion of the leaf traces. It will be remembered that the 

 presence of leaf gaps is a feature of the ferns, in contrast with the other 

 groups of pteridophytes; and their appearance in the dicotyledons is 

 taken to be one indication that this group is connected with ferns, either 

 through gymnosperms or^dire^tly. In tracing the development of 

 the vascular system in a seedling dicotyledon, it is interesting to note that 



the stem cylinder 

 often begins as a 

 protostele, and more 

 or less rapidly be- 

 comes a siphonostele. 

 Monocotyledons. 

 The monocotyledons 

 were once thought to 

 be the primitive 

 angiosperms, but the 

 study of their vascular 

 anatomy has been 

 chiefly instrumental 

 in suggesting the 

 probability that they 

 are derived from 

 dicotyledons. The 

 evidence is obtained 

 from a study of the 

 development of the 

 vascular system from 

 the earliest stages of 

 the seedling to the 

 adult stem. A trans- 

 verse section of an 

 adult stem usually shows " scattered " vascular bundles (fig. 550), quite 

 unlike the arrangement into a hollow vascular cylinder characteristic 

 of the dicotyledons. In studying the development of this stem, how- 

 ever, four stages are often recognized. In the earliest stage the cylinder 

 may be a protostele; and this passes more or less quickly into the second 

 stage, that of the siphonostele, in which the cylinder is just that of a 

 dicotyledon, with its collateral bundles. This means that an embryonic 

 stage of a monocotyledon is the permanent, adult condition of a dicoty- 



FIGS. 547, 548. Tracheids: 547, those of gymnosperms, 

 with bordered pits (after CHAMBERLAIN); 548, the scalari- 

 form tracheids of ferns (after DEBARY). 



