1234 A TEXTBOOK OF THEORETICAL BOTANY 



both guide the pollen tubes chemotropically and nourish them as they grow. 

 The cells are sometimes elongated longitudinally to the style, but not 

 often. The thick walls are soft and in many cases become gelatinized, the 

 cells then appearing to be isolated in a colloidal matrix. The term conduct- 

 ing tissue is however extended to those cases in hollow styles in which it 

 consists only of a modified epidermis or of an epidermis and several sub- 

 jacent layers. All the cells have dense, granular protoplasm and those on 

 the surface become papillose and closely resemble the cells of a stigma. In 

 free carpels superficial conducting layers line the carpel margins and the 

 stvlar groove, when these are stigmatiferous, and extend inwards through 

 the line of the suture to the placentae. 



Conducting tissue strands do not always follow a direct course. For 

 example, in some Rosaceae with laterally attached styles, the conducting 

 strand is continued below the point of attachment of the style and it has 

 been observed that pollen tubes continue their growth downwards to the 

 end of the strand and then turn upwards towards the level of the ovule. 

 In some families the top of the expanded stigma is not functional, and the 

 receptive areas are at, or below, the margin of the stigma cap. Such is the 

 case in Asclepiadaceae, Apocynaceae, many Saxifra^aceae and in Sarracenia 

 and Berberis. How this may have come about does not immediately con- 

 cern us, but it is significant that in many of these styles the conducting 

 strand or its branches are sharply flexed below the apex, which is now 

 passive, and bend outwards and downwards towards the active stigmatic 



surfaces. 



A curious downward development of the conducting strand, from the 

 base of the style into the ovary loculus, is found in Plumbaginaceae, in 

 almost all Portulacaceae, in Euphorbia, Ricinus and in Phytolaccaceae. 

 The majority of these examples have uniovulate locuH and the conducting 

 tissue enlarges in the loculus into a brightly coloured plug which covers 

 the micropyleofthe ovule and is called an obturator (Fig. 1191). Against it 

 the nucellus sometimes grows, into intimate contact. The name is a 

 bad one, for, so far from blocking the micropyle, the obturator acts as a 

 bridge for the pollen tubes from the base of the style to the ovule. 



Styles as we have described them are terminal or dorsal outgrowths, 

 but there exist styles, called gynobasic, which arise from the base, or at 

 least from near the base of the carpel and on the ventral side. The Labiatae 

 and the Boraginaceae possess this character almost uniformly and it occurs 

 sporadically in many other families, such as Ranunculaceae, Rosaceae, 

 Celastraceae and Phytolaccaceae (Fig. 1192). Ontogeny shows that we 

 have here a specialized type of development of peltate carpels. In the 

 earliest stages of the rudiment the single ovule stands on the adaxial carpel 

 sill, not covered by the dorsum, which is at first quite small. Presently, 

 however, the dorsum begins to grow rapidly, developing a hood, from the 

 margin of which arises a vertical style. The hood grows over and encloses 

 the ovule, and its margin, bearing the style, fuses with the placenta. In this 

 way the point of origin of the style is carried over and downwards onto the 



