362 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



assignment to the Bicornes completely untenable and make it seem 

 more probable that its correct place is under the Tubiflorae as a 

 separate suborder occupying a primitive position. 



Cactaceae. The members of this family present a motley assem- 

 blage with every variation from the leafy Pereskia to the tall ribbed 

 columns of Pachycereus, the flat joints of Opuntia, the phylloclades 

 of Epiphyllum, and the tubercled spheres of Mammillaria. There 

 has been considerable divergence of opinion as to its relationships. 

 Wettstein (1935) assigns it to the Centrospermae ; Engler and Diels 

 (1936) to a separate order Opuntiales near the Passifloraceae ; 

 Warming (1904) to the order Cactales following the Centrosper- 

 males; and Hutchinson (1926) to the same order but next to the 

 Cucurbitales (Cucurbitaceae, Begoniaceae, Datiscaceae, and Cari- 

 caceae). 



Practically no embryological work had been done on the Cacta- 

 ceae until the publication of the papers of Mauritzon (1934) and 

 Neumann (1935) on Rhipsalis and Pereskia respectively. Although 

 additional data are desirable, Wettstein 's views have received very 

 definite support from the observations of these authors. 16 The 

 Cactaceae agree with the rest of the Centrospermae in possessing 

 the following embryological characters: (1) anther tapetum glandu- 

 lar and its cells two- to four-nucleate; periplasmodium absent; (2) 

 divisions of microspore mother cells simultaneous; (3) pollen grains 

 trinucleate; (4) ovules campylotropous with strongly curved and 

 massive nucelli; (5) micropyle formed by the swollen apex of the 

 inner integument which protrude out and approach the funiculus; 

 (6) a hypodermal archesporial cell which cuts off a wall cell; (7) 

 formation of a nucellar cap arising from periclinal divisions of the 

 cells of the nucellar epidermis (Fig. 199C); (8) functioning of the 

 chalazal megaspore of the tetrad; (9) formation of a monosporic 

 eight-nucleate embryo sac; 2 (10) functioning of the perisperm as the 

 chief storage region. 3 



An additional point of considerable interest is the occurrence of a 



16 See also Buxbaum (1944, 1948, 1949) who agrees that the Cactaceae are 

 closely allied to the Aizoaceae and should be placed under the Centrospermales. 



2 Archibald (1939) reports an Allium type of embryo sac in Opuntia aurantiaca, 

 but her figures are not convincing and it seems probable that the development is 

 really of the Polygonum type as in the other Cactaceae. 



3 The only exception in this respect is the family Thelygonaceae, in which the 

 endosperm forms the chief storage tissue (Woodcock, 1929). 



