I2O Elementary Biology' 



concentric, seeing that the xylem is internal, and that the 

 phloem encloses it. 



It will be necessary at this point to investigate the struc- 

 ture of the growing point of the rhizome in order to gain 

 some idea as to the mode of origin of these various tissues. 



FIG. 48. Pteris aqnilina.- S - AI ARIFORM VESSEL. (Thome.) 



The growing point of the stem consists of one apical 

 cell, full of protoplasm, the parent of innumerable cells 

 surrounding it, and together with them forming the growing 

 point of the rhizome These growing cells go collectively 

 by the name of meristem. The growing apex is carefully 

 protected by a series of loosely arranged ramenta, but 

 possesses no distinct cap, such as we shall find true roots 

 have. No differentiation of the cells into fibres, vessels, 

 sclerenchyma, &c. is observable near the apex. At some 

 distance from the growing point the cells will be seen to 

 become elongated, thickened, and otherwise metamorphosed 

 to form those modified cells described as composing the 

 fibro-vascular strand. 



It is possible, by prolonged maceration in water, or very 

 dilute potash, to obtain a skeleton of the fibro-vascular system 

 of the fern rhizome. The skeleton forms a very perfect net- 

 work, or netted cylinder, from which smaller strands pass 

 off into the leaves. 



Roots. Passing now to the roots we find that they are 

 given off from the rhizome behind the growing point, and 

 consist primarily of a group of actively dividing cells. The 

 root is covered terminally by a root-cap, which originates by 

 the subdivision of a cell segmented off from the apical 

 cell, whilst other cells, likewise formed from the apical cell, 

 become epidermis, fundamental tissue, and fibro-vascular 



