148 



BOTANY 



VEGETATIVE or growing point is then developed, usually at the 

 apex, and in the simpler cases this consists of but a single cell (Fig. 5). 

 The apex assumes more and more the character of an APICAL CELL from 

 which all the organs of the plant take their origin ; thus, in the case of 

 Cladostephus vertkillatus (Fig. 7), the many-celled main axis terminates 

 in a single conical cell which, by transverse and longitudinal divisions, 

 gives rise to the cellular system of the whole plant. Its side branches 

 are likewise formed from similar apical cells, which develop, in regular 

 acropetal order, from certain of the lateral cells of the parent stem, 

 and determine the character of the branching, to which reference is 

 made in the specific name of this sea-weed. Flat, ribbon-like plants 

 also, such as Dictyota dichotoma (Fig. 8), may have conical but 

 correspondingly compressed apical cells (Fig. 160, A), from which 

 segments are cut off by concave cross walls, and become further 

 divided by subsequent longitudinal walls. The dichotomous branching 



Fig. 160. — Apex of Dictyota dichotoma, showing in A, U, C, D successive stages in the bifurcation 

 of the growing point ; a, apical cell. (After Naget.i.) 



so apparent in Dictyota is preceded by a longitudinal division of the 

 apical cell into two equal adjoining cells (B, a, a). By the enlarge- 

 ment and continuous division of these two new apical cells the now 

 bifurcated stem becomes prolonged into two forked branches (Fig. 

 160, D). In other ribbon-like Algae, on the other hand, and in 

 similarly shaped Hepaticae, as in Metzgeria and Aneuru, the apical cell 

 is wedge-shaped (Fig. 161), and the successive segments are cut off 

 alternately right and left by intersecting oblique walls ; from these 

 segments the whole body of the plant is derived by further division. 

 The apparently strictly dichotomous branching of Hepaticae provided 

 with such apical cells is in reality due to the early development of 

 new apical cells in young segments (Fig. 161, J). In the case of the 

 erect radially symmetrical stems of the Musci, most Ferns and 

 Equisetaceae, the apical cell has generally the shape of an inverted 

 pyramid with plain sides and a convex base, and forms the apex of 

 the vegetative cone characteristic of the more highly organised plants. 



