20 Plant Morphology. 



II. THE WHOLE PLANT OF THYME-LEAVED SANDWORT 



{Arenaria serpyllifolia). 



The plant is a diffusely branched annual. 



The oldest flower stands in the fork of two branches. This 

 flower terminates the stem or properly the rhachis, and branches 

 arise from the axils of the opposite pair of leaves immediately 

 below the first flower. Each of these branches is in turn termi- 

 nated by a flower below which is again a pair of leaves. From 

 the axils of these leaves branches again arise. In this case the 

 method of flowering is fundamentally different from that of Clay- 

 tonia in that the order of flowering is from within outward or 

 from above downward. It has been aptly described in the one 

 case as centripetal, in the other as centrifugal. 



The infloresence in Arenaria is determinate: Its flower-cluster 

 is called a cyme. 



Determinate infloresence may be distinguished from indetermi- 

 nate infloresence, in alternate leaved plants by the position of the 

 bracts. 



Sketch the flower-cluster of Arenaria indicating by numbers 

 the order of flowering. 



Indicate the floral plan of the flower also whatever cohesion or 

 adhesion may occur. 



The general forms of flower-clusters depend principally upon 

 four characteristics : 



1 . As to whether the infloresence is determinate or indetermin- 

 ate. 



2. The length of the rhachis. 



3. The regularity or irregularity of branching of the rhachis. 



4. The length of the pedicels. 



