SECT. I 



MORPHOLOGY 



101 



is filled with water, the underlying tissues impart a greenish shimmer to 

 the root ; but if it contains only air the root appears white. The 

 epidermis of fruits, and particularly 

 of seeds, exhibits a considerable 

 variety of modifications in its mode 

 of thickening, and in the relations 

 the thickening layers bear to one 

 another. The purpose of these 

 modifications in the epidermis be- 

 comes at once evident, when it is 

 taken into consideration that, in 

 the case of flowers and seeds, in 

 addition to protecting and enclosing 

 their internal parts, the epidermis 

 has often to provide for their dis- 

 semination and permanent lodg- 

 ment. 



The Vascular Bundle System. 



— The PRIMARY VASCULAR BUN- 

 DLES extend in the form of strands 

 throughout the body of the higher 

 plants. In more transparent stems, 

 such as those of Impaliens parvi- 

 flora, the bundles may be clearly 

 distinguished and their direction 

 followed. The arrangement of the 

 bundles of leaves is apparent from 

 their venation. In many parallel- 

 veined leaves the bundles are easily 

 isolated. This is often done acci- 

 dentally, as when, for example, in 

 picking a leaf of Plantain (Plantago media) a pull is given at the same 

 time. The leaves, and sometimes also the stems of Mosses, are pro- 

 vided with strands of elongated cells, which are termed conducting 

 bundles. These strands consist either of elongated empty elements, 

 which serve as water-carriers, or include also cells with protoplasmic 

 contents which transport nutritive material (Fig. 159). In the leaves 

 these conducting bundles constitute the midrib. They alwa}^ consist 

 of elements devoid of protoplasm, acting only as water-conductors, 

 and of cells provided with living protoplasmic contents, and concerned 

 with the transport of nourishment. 



A high degree of differentiation of the vascular bundles is first 

 attained by the Pteridophytes, which are accordingly designated 

 Vascular Cryptogams. 



In the Pteridophytes, and throughout the higher plants, two distinct 

 portions may be distinguished in a vascular bundle, the TRACHEAL or 



Fig. 116. — Glandular hair from the inflorescence 

 of Dictatnmts Fravinella, in longitudinal 

 section ; to the right a simple hair laid open 

 at the base. (After Rauter, x 220.) 



