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



the case of a hollow floral receptacle (whether inferior ovary or 

 "calyx-tube"), when the apical growth of the axis is arrested, a 

 zone of embryonic tissue lying close behind the apex gives rise to 

 a projecting ring of tissue, which, by continued basal growth, be- 

 comes a tube enveloping the apex of the shoot. (For the case of a 

 gamopetalous corolla, see Fig. 21, p. 37). 



fn the case of a body or a member which assumes a flattened 

 expanded form, the peripheral growth is effected by the marginal 

 cells, which remain embryonic for a longer or a shorter time ; when 

 the body or member has no definite axis of growth {e.g. gametophyte 

 of Coleoclicete scutata, and that of Aiithoceros) the growth is en- 

 tirely marginal ; but where there are special axes of growth {e.g. 

 foliage-leaves, some fern-prothallia, sexual plant of Marchantiaceae) 

 growth takes place both marginally and by means of definite 

 growing-points. 



The occurrence of apical, intercalary, or marginal growing- 

 points is general ; but it is only in a few cases that there is in a 

 member a growing-point which effects growth in thickness. It 

 is, in fact, the general rule that, when a member has attained its 

 definitive size, it does not increase in thickness. But to this rule 

 there are certain constant exceptions. Thus, the stems and roots 

 of perennial Dicotyledons and Gymnosperms, as well as those of a 

 few Monocotyledons and Pteridophyta, grow thicker year by year 

 by means of a layer of embryonic cells, forming a hollow cylinder, 

 and lying at some little distance within the external surface 

 (see Part II.). This layer, unlike other growing-points, does not, 

 however, give rise to secondary members. 



In the apical growing-point of many inulticellular plants, as in 

 those of some Algse (e.g. Sphacelaria, Chara) of the Muscinese, 

 and of most Pteridophyta, there is one cell, situated in the organic 

 centre of the growing-point, and distinguished from the other 

 embryonic cells by its size (Fig. 3 A, C). This is the apical cell. 

 In some cases there is a group of such apical cells (e.g. species 

 of Selaginella). In these cases, all the new cells formed in the 

 growing-point are derived from the apical cell or cells, from 

 which, as they grow, segments are continually being cut off by 

 cell-division. 



The length of time during which a growing-point remains em- 

 bryonic is not the same in all cases. It may either persist through- 

 out the life of the plant, as is often the case in primary shoots 

 and roots, when the growth of the member is said to be unlimited; 



