THE OVULE 271 



lacking, of ovule traces leading from ventral bundles to the position of 

 lost ovules — Caltha, TroUius (Fig. 42). Where there is a pair of sur- 

 viving ovules — Hydrastis, Calycanthus, many Proteaceous genera — one 

 ovule persists from each submarginal row, as evidenced by tlie origin of 

 its trace. In many achenes, the single ovule has become subbasal — has 

 moved phylogenetically from a higher position during the shortening of 

 the carpel. Evidence of this change of position is in the course of the 

 ovule ti-ace, which extends from the ancestral position of the ovule and 

 bends back to the present position, as in Piper, Ranunculus, the 

 Urticaceae. 



Ontogeny of the 0\tjle 



The ovule arises on the placental surface as a hemispherical projec- 

 tion, which is initiated by cell multiplication, chiefly in the fost and 

 second cell layers beneath the epidermis. The initiating cells may be 

 many or few — one to three in very small ovules. In early stages, divi- 

 sions in the epidermis are largely or wholly anticlinal and, in some 

 taxa, remain so. 



The ovule of some orchids consists of a file of cells formed by one 

 initial covered by the epidermis. Ovules of the Balanophoraceae (Fig. 

 109), Monotropa, and perhaps other taxa may be even simpler. No 

 ovules, as such, are formed in some parasitic families. Continued cell 

 multiplication at the distal end of the primordium builds up a more or 

 less elongate structure, which early shows, near its enlarging tip, evi- 

 dence of archesporial cells. Differentiation within the ovule is basipetal 

 — in sequence, the distal nucellar tip, the inner integument, the outer 

 integument. Rarely, the outer integument precedes the inner in forma- 

 tion. The integuments are built up by annular meristems; the nucellus 

 is enlarged chiefly by proliferation of the hypodermal initials, but, in 

 some taxa, the epidermis plays an important part in the later stages of 

 development by tangential as well as anticlinal divisions. In some 

 families, such as the Vitaceae and Elaeagnaceae, a prominent epidermal 

 cap or calotte is formed in this way, which covers the entire distal end 

 of the nucellus, often appearing sharply distinct from the rest of the 

 nucellus. This cap has been described as the megasporangium wall, 

 with the implication that the rest of the nucellus is potentially sporog- 

 enous tissue. The term calotte has also, unfortunately, been applied to 

 the parietal tissue above the archesporium, where the fertile tissue is 

 deeply sunken; in these nucelli, both an epidermal cap and a calotte 

 may be present. The archesporial tissue is formed from hypodermal 

 cells directly below the epidermis or at some distance below this layer. 

 The cells between the epidermis and the archesporial cells, the covering 

 or parietal cells, may also multiply and aid in increasing the size of the 



