APPENDIX A 645 



away in one piece. Comparison with related plants shows that the large 

 posterior petal is really the equivalent of two obliquely posterior petals fused 

 together. In front view the petals are marked by lines, or honey-guides, con- 

 verging to the centre of the flower (Fig. 495, 1.). 



Androecium, stamens 2, epipetalous, diverging widely right and left. Com- 

 parison shows that these correspond to the two obliquely posterior stamens, 

 while those obliquely anterior are abortive, as well as the median posterior 

 stamen (Fig. 495, 1., in., vn.). 



Gynoecium, carpels 2, antero-posterior, syncarpous, superior. The single 

 style projects between the diverging stamens, and bears a capitate stigma. 

 Ovary bilocular, ovules numerous, placentation axile. 



Fruit, a dry capsule. 



Pollination. The flower is a specialised and reduced example of the Scro- 

 phulariaceae. The change from a pentamerous to an apparently tetramerous 

 type follows from the abortion of the posterior stamen, which leads to abortion 

 of the posterior sepal and fusion of the two obliquely posterior petals. The 

 obliquely anterior stamens are also abortive, not being necessary for effective 

 pollination by flies. The rotate corolla is expanded in a vertical plane, with 

 the style and two stamens projecting horizontally. The insect alighting 

 on the flower gains a foothold by grasping the stamens, drawing them together 

 so that they deposit pollen on the under surface of his body. On going to 

 another flower the stigma receives this, before an additional supply from that 

 flower can be deposited. The result is a high certainty of cross-pollination, 

 with high improbability of self-pollination ; and it is effectively carried out 

 without the three stamens that are abortive. 



ORDER : VERBENALES. 



_ 



Family : Labiatae. Examples : Dead-Nettle, Sage. 



(40) The Labiatae are a very large Family including herbs and shrubs 

 spread through warm and temperate regions, and characterised by their 

 four-angled stamens and decussate leaves. They have often an aromatic 

 smell : Mint, Sage, and Lavender are examples. Their floral structure is 

 very constant. The flowers are either solitary or in axillary cymes. The 

 White Dead Nettle (Lamium album, L.) illustrates the leading features. Its 

 flowers, which are in crowded " verticillasters," show their cymose arrange- 

 ment by the fact that the flower directly in the axil of the leafy bract opens 

 first, and those right and left successively later. Each flower is strongly 

 zygomorphic, with its median plane vertical (Fig. 496). It consists of : 



Calyx, sepals 5, gamosepalous, inferior, odd sepal posterior. 



Corolla, petals 5, gamopetalous, inferior ; strongly two-lipped. One large 

 petal forms the anterior lower lip, two smaller petals guard the entrance to 

 the tube laterally, the upper lip forms a hood, composed of two obliquely 

 posterior petals. The corolla is easily removed in one piece, its tube is narrow 

 below, but widens upwards. 



Androecium, stamens 4, epipetalous. The fifth posterior stamen is absent, 

 its place being inconveniently behind the style. The anthers, of unequal 

 length and opening downwards, lie below the hood of the corolla. In this 

 Family sometimes the outer, sometimes the inner pair are the longer. 



