1030 A TEXTBOOK OF THEORETICAL BOTANY 



they are frequented by honey bees but in other cases only by flies and ants, 

 which may in this way be diverted from the flowers, where their visits might 



be harmful. 



The ground tissue of the petiole is parenchymatous, often with a super- 

 ficial zone of collenchyma, or, in Monocotyledons, of sclerenchyma. The 

 pulvinus, when present, is simply a swollen leaf base and differs only in the 

 relatively greater amount of parenchyma present. Its vascular anatomy is 

 similar to the rest of the petiole. Pulvini which execute movements, such 

 as those of Mimosa piidica (see Volume III), usually show some difference 

 in the parenchyma on their upper and low^er sides, that on the side which 

 becomes concave on stimulation being thinner walled than that on the other 

 side. There are also numerous pits present and protoplasmic connections 

 between the cells. Strictly speaking the term pulvinus, though generally 

 applied to the swollen leaf base, includes nodal swellings on the stem, especially 

 where these are the seat of movement, as in Polygonaceae and Gramineae. 



The vascular strands of the petiole are not as a rule segregated into a 

 distinctive stele, though an endodermis may be present and is sometimes, 

 as in Primula, Valeriana, and Plantago, continued into the lamina, as we have 

 mentioned before. 



It must be remembered that the whole leaf rudiment is essentially a 

 dorsiventral structure and this symmetry persists, not only in the lamina 

 but also in the petiole, which is morphologically a lower portion of the midrib 

 on which the ventral {i.e., adaxial) layer of the rudimentary tissue, that which 

 higher up forms the lamina, has been arrested at an early stage. The petiole 

 therefore, like the midrib, belongs to the dorsal side of the leaf, and its 

 vasculation is dorsiventral, and represents a modified leaf type, not a stem 



type. 



When the lamina has pinnate venation, with a single mam vem, the number 

 of bundles in the petiole is small, normally one to three (Fig. 1021). Where 

 there is only one it may be expanded laterally into a broad band (Fig. 1022). 

 In palmately veined leaves the number may be greater, but in all normal 

 cases the course of the bundles is parallel and they form an arc, open on the 

 upper side. This is the fundamental petiolar type, corresponding to the arc 

 structure of the petiolar traces in the Pteridophyta. A circular radial arrange- 

 ment of the trace bundles is found in the middle portion of the petiole in 

 certain plants, notably in Fatsia (Fig. 1023), but it is a secondary derivative 

 arrangement, produced by the branching and displacement of bundles in 

 the arc, and the normal arc arrangement is seen at the base and top of such 

 petioles, where the dorsiventral character is manifest. 



It has been claimed, on the basis of comparative studies, that the three- 

 bundle type of petiole is primitive among Angiosperms and that the single- 

 bundle type, associated with pinnate venation, is derived from it. The 

 suggestion is that originally the three traces passed directly and separately 

 into the broad base of the lamina (Fig. 1024), but that the greater flexibility 

 attainable with a petiole led to the lateral contraction of the base of the lamina 

 and the association of the three traces close together, leading to the eventual 



