848 



THE HIGHER DICOTYLEDONS. 



The stems are square and hollow ; the leaves arise from the 

 flat sides of the stem in crossed pairs, those on the under- 

 ground parts being small white scales. The aerial shoots are 

 rooted and branched at the base, and 

 then grow erect (6 to 18 inches high). 

 The leaves are all stalked and heart- 

 shaped, the lower ones having longer 

 stalks and broader blades than the 

 upper ones (why is this advantageous 

 to a plant whose leaves are in super- 

 posed rows?). 



The flowers, produced nearly all the 

 year round, but chiefly in summer, are 

 in ring-like clusters round the stem just above a pair of leaves, 

 on the upper part of the shoot. 



Examine several clusters and note that each consists of two 

 groups of flowers, one group in the axil of each leaf ; also that 

 the central flower of each group is the oldest (the first to 

 open), the next oldest being at either side of it, and so on. 



Fig. 140. Diagram indicating 

 the relation of Flowers in 

 half of a cluater (" Verticil- 

 laster"). 



Fig. 141. Dead-nettle, showing two Clusters of Flowers. 



Each group is a biparous cyme which becomes uniparous 

 after the first branching. This is obscured by the fact that 

 the flowers are practically sessile, but it can be made out in 

 Labiates whose flowers have distinct stalks, e.g. Calamint, 

 Black Horehound. Fig. 140 is a diagram of a cyme (half- 

 cluster) ; the main axis (1) ends in a flower and gives rise to 



