312 MORPHOLOGY OF THE ANGIOSPERMS 



The absence of an early free nuclear stage sets the angiosperm embryo 

 well apart from that of the gymnosperms. Primitive form is difficult to 

 determine in the great variety of proembryos in the angiosperms. The 

 presence of suspensors naturally suggests comparison with lower taxa, 

 where suspensors play an important part in embryogeny, the transfer of 

 the young embryo from a superficial to a deep-seated position in the 

 nutritive gametophytic tissues. In the angiosperms, the suspensors play 

 a similar, but less important, part, because the zygote is already close 

 to and sometimes surrounded by the nutritive tissues. Progressive stages 

 in the loss of the suspensor apparently exist. The great diversity in de- 

 tails of form of the early embryo is no greater than that in parts of the 

 mature sporophyte — flower, leaf, ovule — and a correlation between 

 what may be considered advanced or primitive embryo types with ad- 

 vanced or primitive taxa is hardly to be expected. Correlation can per- 

 haps be seen in the presence of the reduced Piperad embryo in the 

 parasitic Balanophorales and Santalales, with their reduced vegetative 

 bodies. 



Comparison of Proenibi-yo Types. In comparison of types of early 

 embryo development, emphasis has been placed on the degree of de- 

 velopment of the suspensor. The suspensor is looked upon as a primi- 

 tive feature of the embryo, probably because of its prominence in lower 

 groups, especially the higher gymnosperms, and a series in reduction 

 can perhaps be found within angiosperms, from massive, many-celled 

 suspensors to those of one or two cells. The massive proembryo is com- 

 monly believed to be the primitive type. Differences in the embryos of 

 dicotyledons and monocotyledons were long ago believed to appear in 

 early-proembryo stages. It has been claimed that, if the longitudinal 

 division of the terminal cell of the young suspensor forms two equal 

 cells, two cotyledons are formed; if these two cells are unequal, only 

 one cotyledon is formed. This conclusion has not been supported. (For 

 comparative morphological study, the embryo of both mono- and 

 dicotyledons must be morphologically mature — not merely at the stage 

 found in the ripe seed before after-ripening; errors of interpretation 

 have been made by the study of immature embryos — those in the seed 

 at time of shedding.) 



In general form and in the possession of a suspensor, the angiosperm 

 embryo resembles that of gymnosperms. In the angiosperms, the sus- 

 pensor seems to be a disappearing structure. In the gymnosperms, the 

 suspensor carries the proembryo from the peripheral position of the 

 archegonia into the central part of the endosperm; in the angiosperms, 

 the suspensor similarly aids in the transfer of the zygote or young 

 embrvo from a position at one end of the sac to a more nearly median 

 position, but the suspensor is a weaker structure than that of gymno- 



