3.] CHAPTER I. GENERAL MORPHOLOGY. 15 



at first in all dimensions and undergoes repeated division in two 

 or three perpendicular meridian planes, thus giving rise to a body 

 which is either a flattened expansion (e.g. species of Coleocheete) ; 

 or a mass of cells (e.g. Volvox, Fucus; sporophyte of Mosses and 

 Vascular Cryptogams) ; thirdly, that in which free cell-formation 

 takes place within the spore, the cells eventually forming the 

 body of the embryo (e.g. Hydrodictyon ; female gametophyte of 

 Phanerogams ; sporophyte of Cycadaceaa and of Ginkgo). 



a. The Development of the Primary Members. With regard to 

 thallophytic plants, it suffices to state that the primary member of 

 the thallus takes origin from the spore in one or other of the pre- 

 ceding modes. 



In the case of cormophytic plants, the first step is the differentia- 

 tion of the body of the embryo into primary shoot and the primary 

 root. 



In unseptate plants, such as the Siphonacese (Algse), the process 

 of segmentation into primary shoot and root is simple ; the spore 

 merely grows out into the shoot at one end, and into the root at 

 the opposite end. In septate plants the segmentation is in many 

 cases clearly indicated (as in Fucus, sporophyte of most Vascular 

 Cryptogams, etc.), by the formation of a wall, termed the basal 

 ivall, which divides the spore into two cells. From, one of these, 

 termed the epibasal cell, the shoot is developed ; from the other, 

 termed the hypobasal cell, the root is developed. 



Whatever the mode in which the differentiation of the primary 

 shoot and of the primary root takes place, the relation between 

 them is in all cases such that their longitudinal axes form one straight 

 line, with the growing-point of the shoot at one end, and that of 

 the root at the other. 



The segmentation of the body of the embryo into root and shoot is usually 

 permanent, but in many plants which do not grow attached (e.g. Spirogyra, 

 [Fig. 6] , and other Zygnemeoe) it is onl/ to be observed iu the early stages of 

 development. 



In some cormophytic plants no primary root is developed, e.g. gametophyte 

 of Mosses ; sporophyte of Salvinia and Psilotum, among Vascular Cryptogams ; 

 Woffia arrhiza, Utri.cularia, and Orchidd, among Phanerogams. 



In those cormophytes in which the shoot is differentiated into 

 stem and leaf, the differentiation takes place at an early stage of 

 development. The primary shoot undergoes differentiation into a 

 primary stem, and one, two, or more, primary leaves. A primary 



