THE ANGIOSPERMAE 1405 



however, is sometimes abortive. Before the embryo sac is mature the eight 

 nuclei are in two groups of four at opposite ends of the sac, and one nucleus 

 from each group migrates to the centre to form the polar pair. 



Embryo sacs are classified on two main characters. The first is the 

 number of megaspores concerned in forming the embryo sac and the second 

 is the number of nuclei in the mature sac. 



As we have seen above, a complete tetrad of megaspores is formed in 

 a very large number of species, probably in the majority. Only one of these 

 may develop into an embryo sac, which is hence called a monosporic sac. 

 In a minority of genera the sporogenous cell does not divide at all, but its 

 nuclei divide and it enlarges directly into an embrv'o sac, which is called a 

 tetrasporic sac. 



Putting aside the considerable number of species in which anomalous 

 numbers of nuclei may arise by irregularities in nuclear division or in 

 other ways, which are not of great general interest, there are two main 

 classes of embry^o sacs with an exceptional number of nuclei, those with 

 four and those with sixteen respectively. These constitute definite types 

 with a certain constancy and may characterize whole genera or even whole 

 families. 



The ten tvpes recognized on the basis of these considerations have been 

 fully described by Maheshwari, from whose work the following account is 

 taken. Thev have been named after the genera in which they were either 

 first described or in which they are most characteristically shown (Fig. 1300). 



1. Monosporic, eight -nucleate sacs. The Polygonum type. Four mega- 

 spores are formed, of which the lowest develops. Its nucleus divides thrice 

 and the eight nuclei formed are arranged in the characteristic order. This 

 is generally referred to as the " normal " type. The name of Polygonum has 

 been attached to it because it was first described in P. divaricatum by 

 Strasburger in 1879. 



2. Monosporic, four-nucleate sacs. Th.t Oenothera ly^&. Four megaspores 

 are formed of which the micropylar one normally develops, though occa- 

 sionally the chalazal spore may also develop. The spore nucleus divides only 

 twice and the four nuclei formed provide the oosphere, two synergidae and 

 one polar nucleus. This type is only known in the Onagraceae and is 

 characteristic of that family. 



3. Bisporic, eight-nucleate sacs. The Allium type. The sporogenous cell 

 divides only into two, a dyad. Either the lower {Allium) or the upper 

 cell {Scilla) may develop. Its nucleus divides thrice and the eight embryo 

 sac nuclei are normally arranged. This type is found sporadically in several 

 monocotyledonous families, e.g., Liliaceae, Aman,'llidaceae and Orchida- 

 ceae, and is widespread in some others, e.g., Alismaceae and Butomaceae. 

 Among Dicotyledons it has only been found in members of certain aberrant 

 families: Podostemaceae, Balanophoraceae and Loranthaceae. 



4. Tetrasporic, eight-nucleate sacs. The Adoxa type. The nucleus of the 

 sporogenous cell divides thrice, or to describe it in another way, the nucleus 

 of the sporogenous cell divides twice to give four megaspore nuclei, without 



