424 



BOTANY. 



About this time there is in most cases a 



Pig. 310. Development of the embryo of Capsella 

 Bursit-pastoris (highly magnified). /., v. uuspensor, or 

 pro-embryo of five cells, and terminated by a four-celled 

 embryo ; 1-1, the longitudinal wall which divided the 

 first embryo-cell into two cells; 2-2, transverse wall 

 which divided each cell of the two-celled embryo, mak- 

 ing it four celled. //., , suspensor; h, the hypophysis, 

 the basal part of the embryo formed by the division of 

 the end cell of the suspensor ; the shaded portion of the 

 embryo is the dermatogen or primary epidermis ///., 

 embryo further advanced ; the inner shaded cells con- 

 stitute the plerome, between these and the dermatogen 

 to the right and left are the cells of the periblem ; the 

 hypophysis is divided into two cells, h, h'. IV., still 

 older condition. F., embryo considerably advanced ; 

 c, c, cotyledons ; , apex of stem ; the dermatogen, peri- 

 blem, and plerome shown as before ; w, the rudiment- 

 ary root, and root-cap formed from the cell h' of ///. 

 and IV. After Hanstein. 



slight differentiation 

 of the inner cells, 

 foreshadowing the 

 future tissue sys- 

 tems (///. and 

 IV., Fig. 310). A 

 little later the cot- 

 yledons (one or 

 two) appear ; in 

 the Monocotyle- 

 dons, in one side of 

 the thallus - like 

 structure a depres- 

 sion forms, which 

 becomes the punc- 

 tum vegetationis of 

 the embryo, and 

 marks the limits of 

 the stem and single 

 cotyledon ; in the 

 Decotyledons two 

 cotyledons grow 

 out symmetrically 

 from the distal end 

 of the thallus-like 

 structure, and the 

 depression between 

 them becomes the 

 punctum vegeta- 

 tionis ( V., Fig. 

 310). The root is 

 the last portion of 

 the embryo form- 

 ed ; its cap (the pil- 

 eorhiza) is develop- 

 ed from a layer of 

 cells resulting from 

 the successive fis- 



sion of the penultimate cell of the suspensor, the hypophy- 



