PART II. VEGETABLE HISTOLOGY. 



differ from those of flowering-plants in the fact that they increase 

 in length by the division of a single sub-apical cell instead of a 

 mass of such cells. The roots of Lycopodiacese differ from those 

 of Ferns and flowering-plants in the fact that their mode of 

 branching is dichotomous. In the higher plants and in Ferns 

 root-branches always originate as lateral outgrowths. 



Besides the differences already mentioned between the roots 

 of Monocotyledons and those of Dicotyledons, another may be 

 noted, namely : the endodermis in Dicotyledons very seldom 

 becomes thick-walled, while 

 the reverse is commonly 

 true of Monocotyledons. 



The tissues of roots are 

 usually somewhat less com- 

 plex than those of the stems 

 of the same plants. 



It was seen in Part I that 

 roots usually branch irregu- 

 larly, while stems branch 

 regularly. The branches also 

 differ from those of stems as 

 regards their origin. While 

 those of stems originate ex- 

 ogenously, or from superficial 

 tissues, those of roots are 

 formed endogenously, or from 

 sublying tissues. They ordinarily spring from the pericambium 

 layer, just in front of a xylem ray of the primary bundle. 



Plants below vascular cryptogams do not possess true roots, 

 but many of them produce outgrowths of much simpler struc- 

 ture, destitute of vascular tissues, which serve the purpose of 

 securing the plants to the soil or rocks. Such organs are called 

 rhizoids. 



Histology of the Stem. A thin, vertical section through 

 the end of a very young stem shows much the same arrangement 

 of tissues as is seen in the root, save that the growing point is at 

 the apex instead of just back of it, consequently no covering of 

 older cells analogous to a root-cap is present. 



The epidermis presents no marked differences from that of 



Fig. 441. — Bi-radial fibro-vascular bundle 

 of the young root of the common Beet, a, en- 

 dodermis ; b, one of the phloem masses ; c, cam- 

 bium cells in the act of fission; d, a duct of 

 the xylem. Magnified about 200 diameters. 

 After Van Tieghem. 



