THE ANGIOSPERMAE 1473 



The Rubiaceae, the Crassulaceae, and the Papaveraceae provide many 

 examples of suspensor haustoria ; but the case of Tropaeolum majiis deserves 

 more than a mention (Fig. 1351)- The embryo sac penetrates the chalaza 

 and forms a large chalazal sac in which the embryo, with its massive cotyle- 

 dons, develops. From the base of the suspensor two haustoria arise. The 

 first grows out through the micropyle and passes over the outside of the 

 ovule, eventually penetrating the ovary wall. The second is directed into 

 the funicle where it enters a cleft alongside of the funicle bundle and this 

 it follows for some distance into the placenta. 



Finally we would call attention to the case of Sempervivum, where 

 haustorial processes are produced by the zygote before division or even by 

 the oosphere before fertilization and reach an extensive development in 

 the nucellus and integuments before embryo formation begins. 



The occurrence of more than one embryo in a seed, a condition called 

 polyembryony, has been known and remarked since pre-scientific 

 times. Many of such cases owe their origin to apomixis and will be dealt 

 with later. Two or more embryos may, however, arise in the same embryo 

 sac as a result of fertilization and this is quite a distinct condition. It may 

 originate in several ways (Fig. 1352). In some cases it arises from the divi- 

 sion of the zygote itself but the number of known examples is not large. 

 The first to be revealed was Erythronium americanmn and similar condi- 

 tions have later been found in Tiilipa gesneriana, Limuocharis emarginata, 

 Vincetoxicum nigrum and V. officinale and in Habenaria platyphylla. Whether 

 it is of constant occurrence in all these species or not, is uncertain. The basal 

 cell of the two-celled zygote divides repeatedly to form an irregular mass ot 

 cells called an " embryogenic mass", and several individual cells of this 

 mass may produce embryos which are twins of the normal embryo. In 

 Habenaria it is certain that only one of these embryos survives in the mature 

 seed. In other species, twin embryos may occasionally survive. 



True cleavage polyembryony, due to the splitting of the zygotic embryo 

 into two or three at an early stage, has been recorded as an abnormality 

 in several species, e.g., Empetrum nigrum, but it occurs more regularly in 

 some of the Orchids and in Linum usitatissimum. It is a rare phenomenon 

 in Angiosperms. Budding of accessory embryos either from the primary 

 embryo itself {Eulop/iea in Orchidaceae) or from suspensor cells {Lobelia 

 syphilitica) is probably only an abnormality. 



Secondly, accessory embryos may originate from other cells of the 

 gametophyte in addition to the normal oosphere. Double oospheres have 

 been recorded but their development is not known. More frequently, one 

 or more synergidae may develop into embryos, either after fertilization by 

 a second pollen tube, or without fertilization, as happens in Linum usitatissi- 

 mum, where in certain strains a very small percentage of haploid-diploid 

 twins occur. Other recorded instances are : Poa alpina, Allium odorum, 

 Iris sibirica and Lilium martagon as well as several of the apomictic genera, 

 e.g., Hieracium. 



The antipodals miay occasionally also be fertilized and form accessory 



