THE EMBRYO AND THE SEED 313 



Accordingly its position was denned by the first segmentations of 

 the zygote. Thus the embryo of a typical Dicotyledon springs from 

 the two distal cells of the filamentous pro-embryo. The larger part, 

 including the cotyledons, plumule, hypocotyl, and most of the root, 

 arises from the distal cell ; the tip of the root originates from the 

 cell next below it ; the rest of the pro-embryo acts as an organ of 

 attachment. 



While the Capsella-type shows the embryogeny usual in Dicotyledons, 

 aberrant forms are not uncommon. But as they are mostly sporadic in their 

 distribution they do not suggest any consistent basis for morphological argu- 

 ment. It is rarely that a family includes many aberrant types : an exception 

 is seen in the Leguminosae, where the peculiarities are most marked in the 

 suspensor. Of the rest, the most interesting variants are the pseudo-mono- 

 cotyledonous embryos. In certain plants that are clearly Dicotyledons in 

 their general characters, only one cotyledon appears. This is seen in Carum 

 bulbocastanum, Eranthis hyemalis and Cyclamen persicum ; and it is probably 

 due to abortion of one of the cotyledons. But in some cases it is believed to 

 result from a lateral fusion of the two cotyledons to form one, as in Ranun- 

 culus Ficaria. Much other evidence suggests that the Monocotyledonous 

 state is derivative from that of the Dicotyledons. This conclusion is further 

 countenanced by the fact that occasionally Monocotyledons are found with 

 two cotyledons (Agapanthus), or even four growing points may appear on a 

 peripheral zone (Cyrtanthus) . In such cases the apex of the axis would be 

 central and terminal, as in the Capsella-type. But in most Monocotyledons 

 it is lateral, as it is in Alisma. 



The development from the zygote in Alisma or Sagittaria is held as 

 typical for a large number of Monocotyledons. The first divisions give 

 rise to a three-celled pro-embryo, of which the basal cell (q) is enlarged, 

 and does not divide further (Fig. 232, i.). The middle cell divides 

 into several cells (m, n, 0, p, Fig. 232, ii.). The distal cell divides 

 into quarters, and subsequently by anticlinal and periclinal walls so as 

 to form the single terminal cotyledon (Fig. 232, ii.-v., /). Meanwhile 

 in the tier of cells below this (Fig. 232, ii. to v., m) a lateral depression 

 begins to appear, which develops into the apex of the axis. This is 

 then lateral in origin, while the cotyledon is terminal. The hypocotyl 

 and root-tip spring from the next two tiers (n-o, Fig. 232, ii.-v.). It 

 thus appears that the reference of the several parts to cells of the 

 pro-embryo differs in the Alisma-type from that in the Capsella-type. 

 The most remarkable difference lies in the lateral origin of the apex 

 of the axis in Alisma, while in the Capsella-type the cotyledons are 

 lateral, and the apex distal. 



In a number of Monocotyledons the embryo differs from the Alisma-type. 

 The most interesting are those in which the apex of the axis originates from 



