370 STRASBURGER ON FOREIGN POLLINATION. 



canal from the stigma, grows downwards therein, and as its proto- 

 plasmic contents move forwards, the emptied portion of the tube 

 become closed up in the usual, way by the formation of cellulose 

 plugs. Yet only a small number of pollen-tubes reach the ovary, and 

 of these the further development is soon arrested, no tube growing 

 into the micropyle of an ovule. In Orchis mascula and O. Morio the 

 process is often carried considerably further. Here the Fritillaria 

 pollen-tubes may penetrate more deeply into the ovary, where they 

 form six strings, growing downwards in the placentas. Now in many 

 Orchidese, the ovules themselves are only formed as a consequence 

 of pollination, and it is highly significant that the development of 

 the ovules in the species named was excited by the pollen-tubes of 

 Fritillaria, as if they had been derived from the pollen of the Orchis 

 itself. In one example of Orchis Morio this development went as far 

 as the formation of the inner integument of the nucellus, when the 

 process was arrested by the death of the pollen -tubes. Thus the 

 stimulus only continues so long as the pollen-tubes remain alive, but 

 so long as it lasts it is quite of the same kind as that produced in 

 normal pollination. In no case did a pollen-tube of Fritillaria reach 

 the micropyle of an ovule and complete the fertilising process. 



On the stigmas of other plants, the pollen grains of Fritillaria were 

 unable to germinate and form pollen-tubes. This happened in experi- 

 ments with Narcissus poeticus, Chelidonium majus, Pozonia officinalis, 

 Cheiranthus Cheiri, Lupinus luteus, Lamiwn album, Doronicu?n Par- 

 dalianches, and other species. 



The reciprocal experiments with these plants were limited by a 

 want of material, but it was established that the pollen of Narcissus 

 poeticus and N odorus germinates as little on Fritillaria as the pollen 

 of Fritillaria on the Narcissi named. Orchis Morio on the contrary, 

 in certain cases, formed tolerably good pollen-tubes on the stigma 

 of Fritillaria. From this it would seem as though a reciprocity in 

 the germinating power of pollen-grains might be inferred to hold 

 generally, but experiment shows that such a reciprocity is absent 

 quite as frequently as it is present. Taking all the experiments into 

 consideration, however, Strasburger thinks it follows, with evidence, 

 that the capacity of pollen grains to put forth tubes on foreign stigmas 

 is in no way restricted within the limits of sexual affinity, nor even 

 v/ithin those of relationship. 



Another position he considers to be established is that the proper 

 pollen of a plant is not able to exclude foreign pollen from germi- 

 nating. On the stigma of Scilla hispanica, its own pollen and that of 

 Fritillaria per sica form tubes simultaneously, which grow intermingled 

 downwards in the style. So, on the stigma of Orchis Morio, its own 



Naturalist, 



