1438 



A TEXTBOOK OF THEORETICAL BOTANY 



and has not been proved. Similar bunches of branches on the nucellus occur 

 in Malvaceae and Onagraceae, and over the embryo sac in Betula. 



Fig. 1320. — A and B, Cucurbita pepo. Branching of the pollen 

 tube at the nucellar apex, penetrating the inner integument. 

 (After Loniio.) C and D, Cyclanthera explodens. Transverse 

 sections of the nucellar apex, before and after fertilization, 

 showing pollen tube haustoria in the nucellus. (After 

 Kirkiiood.) 



When the pollen tube has entered the stigma it may encounter the 

 specialized conducting tissue which is found in many plants and assumes 

 several forms (see p. 1233). Where there is a stylar canal, the conducting 

 tissue may be represented only by its epidermal lining, which is rich in 

 protoplasm and often papillose. There is thus an open canal and the pollen 

 tubes grow on the surface of the conducting tissue. In other plants there 

 may be a more massive conducting tissue, several cells thick, which may 

 still leave an open canal, but which frequently fills the canal with a pectin 

 jelly, derived from swollen cell membranes. The pollen tubes then grow 

 through the jelly. Lastly there may be a solid style with an axial core of 

 conducting tissue (Fig. 1321). There are also many solid styles which have 

 no conducting tissue, notably those of the Grasses. 



The conducting tissue is distinguishable by various features, singly or in 

 combination, such as elongated cells, mucilaginous cell walls, richness in 

 protoplasm, or the presence of sugars. We have seen previously that it is 

 probably an upward extension of the placentae into the sterile stylar region 

 of the carpel and it leads therefore downwards to the placentae. It is fol- 



