THE ANGIOSPERMAE 1165 



alterations in the mature structure may arise from relatively slight adjust- 

 ments of the distribution of growth in the rudimentary stages. Many 

 apparently exceptional forms can be traced on analysis to a few simple 

 changes in the distribution of growth rates among different parts. An ac- 

 celeration here, a retardation there and the outcome may seem altogether 

 different. To take an easy example, let us suppose a young flower with 

 five equal petal-primordia. If these grow most rapidly in their upper 

 regions, five separate petals will result. If, on the other hand, their basal 

 zones, where they are in contact, are most active in growth, a five-lipped 

 floral tube will be found. Should they grow separately at first, and there- 

 after the growing zone shift to the bases, linking two rudiments on one side 

 of the flower and three on the other, a two-lipped labiate corolla will be 

 produced. Probably the commonest mode of formation of a floral tube is 

 that of an early union of primordia into a ring, which elongates by inter- 

 calary growth. 



Temporary cohesions, between the edges of valvate sepals and petals, 

 are not uncommon in the bud stages, being subsequently ruptured when the 

 bud opens. The red sepals of Fuchsia are a good example. Haberlandt has 

 shown that these junctions are formed by the suturing of the marginal 

 cells together by means of many minute interlocking processes, without 

 actual tissue union existing. 



The histological structure of petals is, as a rule, simple. The epidermis 

 is often papillate, giving a velvety surface, but does not often produce long 

 hairs. The epidermal cells, even when not papillate, are often covered with a 

 ridged or warty cuticle which produces a matt surface. Glossy petals are, 

 in fact, exceptional. Those of Raniincuhis, so well known to children, owe 

 their sheen to starch grains in the epidermis, which form a light-reflecting 

 layer. Stomata are usually present, though they seem to be incapable of 

 movement. The guard cells are devoid of chloroplasts and the whole 

 structure is often vestigial. The interior tissue is generally loosely organized, 

 with irregular cells and abundant intercellular spaces, and there is rarely 

 any differentiation of a palisade layer corresponding to that in foliage leaves. 

 Secretory cells and epidermal glands are commonly present and secrete 

 the aromatic substances which give flowers their perfume. The walls of the 

 epidermal cells are often very highly convoluted and in many cases the 

 convolutions open into loops, leaving small intercellular passages. The 

 whole outer surface is however covered by a continuous cuticle, which 

 varies in thickness in different species. Chromoplasts may be present in 

 some red and yellow flowers, but in some yellow and in most blue or 

 purple flowers, the cells, including sometimes those of the epidermis, are 

 filled with anthocyan pigments in solution and no plastids occur. 



The vascular supply to the petal is, almost universally, a single trace 

 bundle, but this branches pinnately from near the base, except in very small 

 petals, and the branches form an open fan-pattern, following apparently 

 the marginal expansion of the petal rudiment. Marginal bundles, which 

 are common in leaves, are rare in petals and are confined to a fev^ families 



