SECT. 1 MORPHOLOGY 107 



In stems Avith apical cells the rudiments of new leaves and 

 SHOOTS are developed from single peripheral cells, or cell groups of 

 the vegetative cone (Fig. 113). In such cases, not only the new- 

 shoots, but even the leaves, usually begin their development with an 

 apical cell. The apical cells of the leaves, however, soon disappear, 

 and further growth proceeds along their whole margin. 



In those fern leaves which are able to form terminal buds (p. 31) Goebel C^") 

 has observed the direct transition of the two-sided apical cell of the leaf tip to the 

 three-sided j^yramidal apical cell of the young shoot ; this takes 2)lace by the 

 origin of a suitably placed wall in the apical cell of the leaf. 



In a stem with no apical cells (Figs. 17, 115), the rudiments of 

 the leaves and new shoots first appear as small protuberances, the 

 formation of which is generally initiated by the periclinal division of 

 a group of periblem cells ; while the cells of the overlying dermatogen 

 continue to divide by walls at right angles to the surface. In the 

 case of new shoots developing at some distance from the growing 

 point of the parent stem, the cells from which they are destined 

 to arise retain for that purpose their original embryonic character. 

 The formation of the buds on the rapidly growing shoots of bushes 

 and trees in the spring may be postponed, so that the rudimentary 

 lateral shoots first appear in the axils of the eighth or even the tenth 

 youngest leaves, and consequently at points where the differentiation 

 of the surrounding tissue has already begun Q^^). 



With the exception of those Pteridophytes whose roots as well as 

 stems are dichotomously branched (cf. p. 47) the branches of all other 

 normal roots, whether growing by means of an apical cell or not, arise 

 in acropetal succession ; the branching first begins in regions consider- 

 ably removed from the growing point, where the differentiation of the 

 tissues is already complete. Since their origin is endogenous (j). 47) 

 the lateral roots must push through the whole outer tissues of the 

 parent root. 



Meristem. — An undifferentiated tissue, the cells of which are still 

 capable of division, is termed embryonic tissue, or meristem. The 

 meristem of embryonic rudiments and of the growing point is called 

 promeristem, and all meristematic tissue which can be shown to have 

 been derived directly from such promeristem is termed primary. A 

 primary meristem may still retain its meristematic character in the 

 midst of a mature tissue. Fully differentiated tissue is designated 

 permanent tissue, or somatic tissue, in contrast to meristematic tissue. 

 At times, permanent tissue may again become capable of division, and 

 thus give vise to a secondary meristem. 



Kinds of Tissue. — The matui'e tissues of the plant may be divided 

 into two groups, parenchyma and prosenchyma, between which, 

 however, no sharp distinction can be made. A typically developed 

 parenchymatous tissue is one in which the thin-walled cells are equally 



