202 MORPHOLOGY OF THE ANGIOSPERMS 



pollen tube was seen to enter the stigma and its tip to be present at the 

 ovule ended the pollen-grain-descent theory. The distribution of the 

 transmitting tissue probably led to the discovery of its function. It had 

 been noted also that considerable time elapsed between the presence 

 of pollen on the stigma and the presence of an embryo. Finally, the 

 pollen tube was followed from stigma to ovule, and a "double role" 

 was assigned to the transmitting tissue: "to nourish the tube in its long 

 course" and "to guide it to the ovule." 



Types of Transmitting Tissue. Transmitting tissue is apparently an 

 elaboration of an area of simple, papillose epidermis; the cells separate 

 and break down in part, forming a complex, often deep-seated, tissue. 

 The nonpapillose transmitting tissue of water plants and of some lower 

 monocotyledons probably represents a loss of papillose form in the 

 cells; and the haii'-coated stigmas and styles of the grasses and sedges, 

 a modification related to anemophily. Small, isodiametric cells form 

 the stigmatic surface in many aquatic genera — Najas. In many wind- 

 pollinated plants, the stigmatic surface is extended by hairlike processes; 

 the important "receptive haiis" may be true hairs or merely tips of 

 papillose epidermis cells. In the Liliales and some related taxa, the 

 transmitting tissue is a simple epidermal layer, with few or no papillae. 

 A most extreme form of the nonpapillose type is the multilayered, 

 disintegrating tissue of the orchids. In dicotyledons, the transmitting 

 tissue is usually papillose and reaches its greatest complexity in the 

 gamopetalous taxa, where carpel margins are intimately fused and 

 proliferated. Modifications of vascular tissue accompany unusual forms 

 of the stigma. Where the stigma is simple and not well defined, as in 

 Zea, the vascular tissue becomes gradually weaker, distally; where the 

 stigma is large or lobed, the vascular tissue branches peripherally into 

 many delicate strands. 



Form of the Stigma. Variations in form of the stigma are largely re- 

 lated to the morphological structure of the carpel and gynoecium. A 

 more or less distinctly two-lobed stigma, retaining, in part, the two- 

 lipped form of the primitive ancestral crest, is frequent in less advanced 

 taxa. Anatomically, two-lobed stigmas are of two types: the median 

 vascular bundle dichotomously forked, the lateral bundles shorter; the 

 median bundle short, the laterals prolonged. These vascular variations 

 reflect the fundamental structure of the angiosperm node and ap- 

 pendages — a median trace, double in nature, with a pair of laterals 

 (Fig. 6C) (the "three-trace supply," formerly considered basic for 

 carpels). Two-lobed stigmas seem to be primitive types; the highly 

 specialized stigma is simple — globose or cylindrical — or elaborate in 

 form — plumose or dissected. Greatly increased stigmatic surface is 



