386 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



only about half as heavy as the normal ones. By dissecting out 

 the embryos and placing them on damp blotting paper, he was able 

 to induce their germination, and the resulting plants flowered and 

 fruited abundantly. The reciprocal cross L. austriacum X L. per- 

 enne was more difficult, for here the fruits shed prematurely at a 

 time when the seeds had only one-thirteenth of the weight of normal 

 seeds and were incapable of germination. However, by excising 

 the embryos when they were about a fortnight old and placing them 

 on cotton wadding in little tubes containing 10 to 15 per cent sugar, 

 he was able to promote their growth. A couple of weeks later they 

 were removed from the sugar solution and placed on moist blotting 

 paper, where they germinated within a few days and eventually 

 yielded vigorous plants which flowered and fruited normally. Lai- 

 bach, therefore, expressed the opinion that a similar method of 

 "artificial premature birth" could perhaps be used to obtain off- 

 spring from various other crosses which had so far proved unfruit- 

 ful. In conclusion he said: "In any case, I deem it advisable to 

 be cautious in declaring combinations between higher plants to be 

 unviable after fertilization has taken place and after they have 

 begun to develop. Experiments to bring the aborted seed to de- 

 velopment should always be undertaken, if it is desirable for theoreti- 

 cal or practical reasons. The experiments will not always be suc- 

 cessful, but many a result might be obtained by studying the condi- 

 tions of ripeness of the embryos and by finding out the right time 

 for the preparing out of the seeds." 



Laibach's brilliant exposition gave the lead for more intensive 

 studies on the artificial culture of embryos, and during recent years 

 a number of papers have appeared on the subject. In several 

 crosses which were formerly unsuccessful, the hybrid embryos have 

 been successfully reared to maturity. 



In mentioning some specific cases where this technique has been 

 employed with success, we may first refer to certain stone fruits, 

 such as Prunus avium (sweet cherry), P. domestica (plum), and P. 

 persica (peach). When crosses are made in which the early ripen- 

 ing varieties of these plants are used as female parents, the embryos 

 abort and the seeds are not viable. 3 Tukey (1944) attempted arti- 



3 Histological studies showed that although fertilization takes place normally, 

 the endosperm and embryo soon cease developing, followed also by a collapse of 

 the nucellus and integument (Tukey, 1933). 



