14 BRITISH FLO WEEING PLANTS chap. 



take the position to secure the fullest amount of light. 

 In Fig. 5 this arrangement is clearly shown. Now if 

 we imagine the intervals between the leaves suppressed, 

 we shall see that they would lie in whorls of five. In 

 fact, the five sepals form one such whorl and the five 

 petals another. We may test this suggestion by taking 

 the case of the Lilac and Privet, which have 4 

 sepals and 4 petals. Here the leaves are opposite, 

 each pair at right angles to those above and below. 

 Here, therefore, the whorl of leaves would consist, not 

 of five, but of four. There are indeed some excep- 

 tions which must be otherwise accounted for. For 

 instance, in genera which contain species of different 

 sizes the number of sepals and petals is. often less in the 

 smaller than in the larger ones. 



Flowers with parts in fours are also frequently met 

 with in orders where the pentamerous arrangement is 

 the more common. Caryophyllacese is a good example. 

 Taking our British genera, we find in the smaller 

 flowered ones — such as Cerastium, Stellaria, and 

 Sagina — the number of parts sometimes 4 instead of 5. 

 Assuming, as we may, that all had a common penta- 

 merous ancestor, we regard the tetramerous forms as 

 resulting from the dropping out of one member from 

 each whorl. In other cases, however, the tetramery is 

 due, not to a loss, but to a fusion of parts. Thus most 

 species of Veronica have 4 sepals and a 4-lobed corolla. 

 There is no doubt, however, that the upper petal 

 represents two which have become united. It is very 

 often larger than the others, and sometimes its double 

 origin is indicated by a notch. In some cases, more- 

 over, both petals are developed, and we have a penta- 

 merous corolla, bringing Veronica into line with the 

 other members of its family, Scrophularinese, where the 

 corolla is formed on a pentamerous plan. The tetra- 

 merous calyx is explained by the loss of the upper- 

 most sepal ; in pentamerous species this, however, is 

 developed. In one of our commonest British orders, 

 Cruciferse, the flower is built on a 4-plan, but we 



