lOO 



THE STRUCTURE OF ECONOMIC PLANTS 



lying epidermal cells divide only anticlinally to compensate for 

 the enlargement of the nucellus. As this takes place, an annular 

 fold arises at the base of the nucellus which becomes the inner in- 

 tegument. This usually grows with greater rapidity than the 

 nucellar tissue; and, in most cases, eventually encloses it. When 

 there are two integuments, the outer one is initiated soon after the 

 inner is differentiated and later completely surrounds it. 



In the orthotropous type, the ovule develops as a straight struc- 

 ture; and the integuments grow out and enclose the nucellus in 

 such a way that the microfyle is in a straight line with the chala%a 

 and hilum. This form of ovule is also termed atropous. Common 

 examples occur in the Polygonaceae (including Fagopyrum, buck- 



Fig. 38. Types of ovules: A, orthotropous; B, campylotropous; C, hemitropous; D, 

 anatropous (all diagrammatic): cha, chalaza; /«, funiculus; hi, hilum; mic, micropyle; nls, 

 nucellus; r, raphe. 



wheat, and Rheum, rhubarb), in the Juglandaceae, Urticaceae, and 

 several other families. (Fig. 38, A.^ 



Where the nucellus is not in a straight line with the funiculus 

 but is curved or bent over in such a way that the chalaza and micro- 

 pyle are in a plane at right angles to the funiculus, the type is 

 known as campylotropous. This is common in many of the Caryo- 

 phyllaceae and Berberidaceae. (Fig. 38, B.) Where the funiculus 

 is sharply curved just below the chalaza so that the nucellus and 

 integuments are inverted and lie alongside the stalk of the funiculus 

 with the micropyle directed toward the placenta, the ovule is 

 anatropous. (Fig. 38, D.) 



A fourth type, hemitropous , which is intermediate between ortho- 

 tropous and anatropous, occurs when the ovule is half inverted 

 rather than completely so, as in anatropous ovules, so that the 



