124 



THE BIOLOGY OF A PLANT. 



alternately on its riglit and left sides. These cells in turn con- 

 tinue to grow and divide, and thus give rise to two similar masses 

 of meristem, which together constitute the apical bud. From 

 the meristem by gradual, though rapid, changes the various tis- 

 sues of the adult rhizome are differentiated ; and longitudinal 

 sections passing through the lateral ridges show the mature 

 tissues fading out in a region of indifferent meristem about the 

 apical cell (Fig. 55b). 



The apical cell lies at the bottom of a funnel-shaped depression at the 

 tip of the stem. It is shaped approximately like a thin, two-edged wedge 

 w'ith an arched or curved base turned forwards towards the centre of the 

 funnel-shaped depression. The thin edge of the wedge is directed back- 

 wards, and its sides, which are also curved, meet in a vertical plane above 

 and below. A longitudinal section taken through the plane of the lateral 



a<o 



Fig. 56. (After Sachs.)— A vertical transverse section through the apical cell, a.Ct 

 showing a boundary of hairs and a second apical cell, I, belonging to a leaf. 



ridges therefore shows the apical cell in a triangular form as in Fig. 55b. 

 A section taken at right angles to this — i.e., vertical and longitudinal — 

 shows the cell to be approximately rectangular and quadrilateral (Fig. 

 55a), while a transverse vertical section shows it in the form of a bi-convex 

 lens (Fig. 56). 



The funnel-shaped depression is compressed vertically, and its w^alls are 

 thickly covered with erect branching hairs, w^hich are closely fastened 



m. 



Fig. 57.— Cross-section of an entire fertile leaflet, m.r, midrib ; v, veins ; ep, epi- 

 dermis ; ms, mesophyll ; sp, sporangia ; in, indusium. 



together by a hardened mucilage secreted by the apical bud. These hairs 

 entirely close the mouth of the funnel and shut off the delicate young 



