16 PART I. THE MORPHOLOGY OF PLANTS. [ 3. 



leaf which is thus contemporary in origin with the primary stem 

 is termed a cotyledon. The cotyledons, as embryonic members, 

 differ more or less widely in form from the ordinary foliage-leaves 

 of the adult plant. 



b. Growth. The development of the embryo into the adult or- 

 ganism is effected by the further growth of the primary members, 

 and by the development of secondary members. 



When the adult form is small and lowly organised, the process 

 of growth may consist merely in a slight increase in size of the 

 cell or cells constituting the embryo. But where the adult form 

 is relatively large, the process of growth involves also a consider- 

 able increase in the mass of the protoplasm of the embryo. Thus, 

 in the development of a tree such as an Oak, not only is there an 



Fie. 6. Young plant of Spirogyra, showing temporary differentiation into root and 

 shoot ; the root-end is still in the zygospore (after Pringsheim, x 125). 



increase in size of the cells of the embryo, but there is also a 

 formation of additional protoplasm and therefore also a formation 

 of additional cells. The formation of new protoplasm takes place 

 at first throughout the body of the embryo, that is, inter stitially, 

 the whole protoplasm being in the embryonic condition ; but 

 gradually more or less of the protoplasm passes over into the 

 adult condition, and in those parts no further formation of addi- 

 tional protoplasm or cells takes place. In some cases (e.g. Volvox) 

 the whole protoplasm passes over simultaneously into the adult 

 condition ; but more commonly certain portions of the protoplasm 

 remain embryonic for a longer or shorter time. These persistent 

 embryonic regions are termed growing -points. At first they are 



