THE ANGIOSPERMAE 1465 



The system proposed by Johansen recognized the following types: 



I. The Piperad type. 2. The Onagrad Type. 3. The Asterad Type. 

 4. The Caryophyllad Type. 5. The Solanad Type. 6. The Cheno- 

 podiad Type. 



The names are obviously derived from the names of the families in which 

 the tvpe is most characteristically shown. The only change from Schnarf's 

 Types is the substitution of Onagraceae for Cruciferae in the second Type, 

 since Capsella, which was the basis of the Cruciferae Type, has an embryonic 

 development which so far from being generalized, as was once supposed, is 

 limited to the Brassicae and forms one Variation in the Onagrad Type. 



Before considering the characters of these Types, there are two general 

 comments to be made. One is, that it is principally the pro-embryo that is 

 in question and it is on the early cell-divisions that the Types are based. 

 When exactly the pro-embryonic stage ends and the embryo " proper " 

 begins is not very clear, though the general idea is plain. Schnarf gives the 

 term a very restricted application, limiting it to structures before any divi- 

 sions occur which begin the development of the embryo proper. This stage 

 is generally reached with the first longitudinal division of the terminal 

 cell, which is a precise limitation. Soueges uses the term in a broader sense 

 and classes as pro-embryo all structures formed while the radial symmetry 

 is retained. Embryo formation thus begins for him usually with the initia- 

 tion of the cotyledons, when radial symmetry is lost. Most writers seem to 

 have adopted the latter view. 



The second comment is that the various embryonic types show sur- 

 prisingly little systematic affiliation. Thus under the Onagrad Type are 

 included Variations found in no fewer than nine unrelated families of Di- 

 cotyledons and two families of Monocotyledons. The other Types are 

 almost equally miscellaneous. Even in the same families there is no uni- 

 formity. Onagraceae are, as a matter of fact, fairly uniform, but Solanaceae 

 shows six Variations within the Type family itself. Nor are the smaller 

 units, the Variations, always systematically homogeneous. In most cases 

 they are limited to a single genus or species, but, for example, the Myrio- 

 phxUinn Variation of the Caryophyllad Type includes, besides Myriophyllum 

 itself, Portiilaca, Pyrola, and Samolus. This is not a criticism of the classifi- 

 cation as such, but it does show that embryology has a limited value as a 

 systematic criterion. 



The following are the characteristics of the six embryological Types as 

 defined by Johansen. 



A. Piperad Type. The zygote divides by a longitudinal wall. 



B. The zygote divides by a transverse wall. 



I. In the second cell-generation the terminal cell divides longitudinally. 



Onagrad Type. The basal cell contributes little or nothing to the 

 building of the embryo. 



