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A TEXTBOOK OF THEORETICAL BOTANY 



sac, why do pollen tubes penetrate ovules where the embryo sacs are abortive, 

 as they may do, or even reach the ovary before ovules have been formed? 



We are a long way from a full understanding of this important matter, 

 but the answer will certainly not be a simple one. Perhaps we may conclude 

 that the course of the tube in the style and as far as the placenta is mechani 

 cally determined and that towards the end of its course it comes wathin the 

 influence of a chemotactic substance emanating either from the micropyle 

 or from the synergidae or both, which completes the direction of its growth. 



In the ovary the pollen tube may follow an aerial course to the micro- 

 pyle, but rarely, if ever, over any considerable distance. In anatropous 

 ovules the micropyle lies close to the surface of the placenta and the gap may 

 be filled by mucilaginous secretions from the micropyle. In other cases the 

 pollen tube usually creeps on the surfaces of the placenta, the funicle and 

 the integument to reach the micropyle. A variety of special structures also 

 exist to facilitate its course. The obturators which we have described on 

 p. 1234 are structures of this kind, and the bunch of elongated cells which 

 grow downwards from the base of the stylar canal in Thymelaeaceae, like 

 hairs, directed towards the ovule, are similar in kind. Hairs from the pla- 

 centa, hairs from the funicle, hairs from the integuments, may all be found 

 helping to bridge the gap which must be crossed between the ovary wall and 



Fig. 1322. — Ovular " stigmas " formed by up- 

 growths from the microp> lar endostome into the 

 stylar canal. A, Myiiocaipa longipes. B, 

 Leucosyke capitellnta. {After Fagerlifid.) 



the micropyle. In a few species (e.g., IxioUrion pallasii, Isolepis gracilis, 

 Leucosyke capitellata, Myriocarpa longipes, Polygonum persicaria, etc.) there 

 is such a copious outgrowth of cell filaments from the micropyle, reaching 

 up to, and into, the stylar canal, that one might almost call them ovular 

 stigmas (Fig. 1322). 



