CHAPTERS ON EVOLUTION. 



on their axes. From cases in which we find flowers situated each 

 on a distinct stalk of its own, as in the Corymb and the Umbel (Fig. 

 204) of botanists, to the condition of the " flower-head," we can pass 

 by easy gradations. If we cut short the stalks of the umbel, and 

 thus crowd the separate flowers on the end of a common stalk, we 



obtain a fair idea of the pos- 

 sible origin of a flower-head 

 by abortion of the flower-stalks 

 of an umbel or allied floral 

 arrangement. The fact that 

 such crowding of flower-heads 

 on a common stalk is not limi- 

 ted to the Composites or Daisy- 

 tribe, but occurs in other plant- 

 orders, argues powerfully in 

 favour of its acquired nature 

 as the result of common 

 modifying conditions. Thus 

 a head of clover essentially 



Fro. SOS-HEAD OF TmsTL^sHowiNG NUMEROUS ^^ ^ condkion of ^ 



thistle or centaury. We can 



obtain a fair idea of the effect of modification by the disappearance 

 of flower-stalks, if we look at a simple umbel, seen in the cherry (Fig. 



FIG. 204. a, SIMPLE UMBEL OF CHERRY ; b, COMPOUND UMBEL OF FOOL'S PARSLEY. 



204, a), or a compound umbel, seen in fool's parsley (Fig. 204, b], 

 and, by crowding the flowers together, minus their stalks, imagine their 

 growth in one stalkless group to represent the " flower-head " of the 

 daisy or thistle. 



Summing up our studies in organic individuality, we may say that, 



