CHARACE.E. 



279 



From the central cell of the fruit of Chara^ the sexual leaf-forming plant is not 

 immediately developed, but a Pro-emhryo precedes it, which attains only small dimen- 

 sions and consists of a single row of cells with limited apical growth. The stem of 

 the Leaf-bearing Sexual pla?it springs from a cell which lies at some distance from 

 the apex of the pro-embryo and grows in a direction 

 nearly at right angles to that of its axis. The unlimited 

 apical growth of the plant depends on an apical cell 

 (Fig. 199, C, t) from which segments are cut off by 

 septa. Each segment immediately divides again by a 

 septum into two superposed cells, the lower one of 

 which ig) always grows without further division into a 

 long internode (frequently 5 to 6 cm. in length) ; the 

 upper one scarcely lengthens, but is first divided in 

 half by a vertical wall, and each half then divides by 

 further successive septa so as to form a whorl of peri- 

 pheral cells. From the node thus constituted the 

 leaves are developed, each from a peripheral cell, and 

 the normal lateral branches, which always originate 

 from the axil of the first or of the two first leaves of 

 the whorl. The leaves of such a whorl, from 4 to 10 

 in number, repeat in a modified manner the develop- 

 ment of the stem, but their apical growth is limited. 

 After the formation of a definite number of segments, 

 the apical cell ceases to divide and grows into the 

 terminal cell of the leaf which is usually painted 

 (Fig. 199, A, h"). From these leaves lateral leaflets 

 may arise in a similar manner to that in which 

 the leaves themselves have been formed from the 

 stem ; and the leaflets may again in turn produce 

 others of a higher order. The successive whorls of 

 a stem alternate, and in such a manner that the 

 oldest leaves of the whorl, in the axils of which the 

 branches stand, are arranged on a spiral line winding 

 round the stem. Each internode also usually under- 

 goes a subsequent torsion in the same direction. 

 The lateral shoots, of which in Chara one is always developed in the axil of 

 the oldest, in Nitella one in the axil of each of the two oldest leaves of the 

 whorl, repeat the primary stem in all respects (Fig. 210, p. 287). It has already 

 been mentioned that the leaves undergo a segmentation similar to that of the 

 stem ; they also consist at first of very short internodes which are afterwards 

 greatly elongated (Fig. 199, B, y), and are separated by inconspicuous transverse 

 plates or nodes. From these the leaflets arise in whorls the members of which are 

 formed in succession, but they are directly superposed one above another, and do 



I-IG. 198. — Chara fragilis ; sp germina- 

 tinfj spore; i d q pi together form the pro- 

 embryo (// is segmented, which is not 

 clearly indicated in tiie drawing) ; at d are 

 the rliizoids iv ; w' the so-called primary 

 root; ^ the first leaves (not a whorl) of the 

 second generation or leaf-bearing plant (after 

 Pringsheim, x about 4). 



This has not yet been observed in Nitella. 



