430 



BOTANY 



Each pollen- grain is provided with a delicate wall, which is 

 differentiated into a cuticularised exine, and an intine consisting 

 chiefly of pectose. The surface of the pollen -grain is frequently 

 studded with projecting points or warts, or beset with delicate and 

 regularly disposed outgrowths (Fig. 359, B). In addition, thinner 

 spots (Fig. 359, B) or areas denned by a lid-like covering (Fig. 360, B) 



Fig. 359. — A, Pollen-grains of the Heather 

 (fiaUuna vulgaris), cohering in tetrads ; B, 

 simple pollen-grains of the Lime (Tilia). 

 (x 350.) 



Fig. 360.-^1, Pollen-grain of CucurMta Pepo ( x 240) ; 

 B, section of pollen-grain of Cucurbita verrucosa, 

 showing one of the lid-like areas through which 

 the pollen -tubes protrude ( x 540). 



often occur in the walls of the pollen-grains, they fulfil an important 

 office as germ-pores in facilitating the processes of fertilisation. 



The macrosporangia of Phanerogams constitute the seed-rudiments, 

 and are called ovules. They usually arise on the margins of the 

 carpels, and are either free or entirely enclosed by them. The first 

 case is characteristic of the class of the Gymnosperms ; the second of 

 the Angiosperms. 



An ovule (Fig. 361), when ready for fertilisation, is represented 



Fig. 361.- Diagrams of the ovule. A, Atropous ; B, anatropous ; C, cainpylotropous ; m, mlcropyle ; 

 i, integuments ; n, nucellus ; ch, chalaza ; /, funiculus ; r, raphe. 



by'an ellipsoidal body attached to the carpel, usually by a stalk, the 

 funiculus (/). The central portion of the ovule is occupied by a 



