102 



BOTANY 



PART I 



axis terminates in a single conical cell which, by transverse and 

 longitudinal divisions, gives rise to the cellular system of the whole 



plant. Its lateral 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. 

 Flat, ribbon-like plants also, 

 such asDidi/ota dichotoma (^^^) 

 (Fig. 8), may have conical 

 but correspondingly com- 

 pressed apical cells (Fig. 

 Ill A), from which segments 

 are cut off by concave cross 

 Avails, and become further 

 divided by subsequent longi- 

 tudinal walls. The dicho- 

 toinous branching so appar- 

 ent in Dictyota, and referred 

 to on pp. 13, 17, is preceded 

 by a longitudinal division of 

 the apical cell into two equal 

 adjoining cells {B, a, a). By 

 the enlargement and con- 

 tinuous division of these two new apical cells the now bifurcated stem 

 becomes prolonged into two forked branches. In other ribbon-like 

 Algae, on the other hand, and in similarly 

 shaped Hepaticae, as in Metzgeria and 

 Aneura (^'^^), the apical cell is wedge-shaped 

 (Fig. 112), and the successive segments 

 are cut off alternately right and left by 

 intersecting oblique walls ; from these seg- 

 ments the whole body of the plant is de- 

 inved 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. 112 h). In 

 the case of the erect radially symmetrical 

 stems of the Musci, most Ferns and Equise- 

 taceae, the apical cell has generally the 

 shape of an inverted three-sided pyramid (^*'^) with a convex base, and 

 forms the apex of the vegetative cone characteristic of the more highly 



Fio. 111. — Tlie growing point oi Dictyota dichotoma, sliow- 

 iiig tlie dichotomous branching. A, Initial cell. 

 (After E. de Wildeman, x 500.) 



Fi(i. 112. — Diiignimmatic representa- 

 tion of theapex oi Metzgeria furcata 

 In process of branching, viewed 

 from the dorsal side, a, Apical 

 cell of parent shoot ; b, apical cell 

 of daughter shoot. (After Knv, 

 X circa 370.) 



