150 MORPHOLOGY OF ANGIOSPERMS 



In 1898 Nawaschin 30 described some remarkable variations 

 in the course of the pollen-tube in Ulmus pedunculata and U. 

 montana. In addition to tubes following the ordinary chala- 

 zogamic route, some instead of penetrating the chalaza pass 

 from the funiculus across the short outer integument, and 

 thence into and upward through the inner integument to the 

 top of the nucellus, when they turn across to the bottom of the 

 micropyle and so enter the nucellus from the usual direction ; 

 others follow the same route except that they pass directly from 

 the funiculus into the inner integument ; while still other tubes 

 branch profusely and apparently with no definiteness within 

 both the funiculus and integument. In the same species, there- 

 fore, pollen-tubes may enter the sac either at the antipodal or 

 micropylar ends, and may either pass with great directness or 

 branch profusely. 



The behavior of the pollen-tubes in Ulmus suggested that 

 there might be other routes than through the micropyle or 

 through the chalaza, and this has been observed in other forms. 

 In his study of the Cannabineae in 1898, Zinger 31 discovered 

 that the two thick integuments completely coalesce over the 

 apex of the nucellus, and the micropyle is entirely closed by 

 tissue. The pollen-tube either bores its way through the tissue 

 filling the micropyle or pierces the two integuments, reaching 

 the nucellus and branching about its apex, and finally sending 

 one very slender branch into the embryo-sac. 



With these facts before them, Pirotta and Longo 41 pro- 

 posed the term " acrogamy " for the entrance of the pollen-tube 

 directly through the micropyle ; " basigamy " for its entrance 

 through the chalaza (Casuarina, Betula, Alnus, Corylus, Carpi- 

 nus, Juglans, and sometimes Ulmus) ; and " mesogamy " for 

 its entrance by intermediate routes (sometimes Ulmus, and 

 Cannabineae). In the following year Longo 49 described a case 

 of mesogamy in Cucurbita, in which the pollen-tube traverses 

 the tissues of the funiculus and outer integument before enter- 

 ing the micropyle. Practically the same phenomenon has been 

 observed by Murbeck 50 in Alchemilla arvensis, in which the 

 micropyle is entirely closed by the growth of the integument, 

 and the pollen-tube enters the ovule at the chalazal end, trav- 

 erses the entire length of the integument within its tissues, and 

 thus enters the micropylar extremity of the embryo-sac. 



