ROOTS OF CERTAIN PALMS. 281 



greater differentiation and integration; and the problem changes as some of the 

 functions become chief duties of the older parts. 



A young root is long and slender, exposing a relatively large surface favourable 

 to absorption and respiration ; the cylindrical form exhibits this surface on all sides, 

 and gives the concentration of bulk, which combines strength with a slenderness and 

 flexibility that facilitates insinuation. 



Length of root necessitates an efficient system of transport, in which the passage alike 

 of crude and elaborated materials is protected from interruption due to bending or to 

 pressure from without. The pressure which must sometimes act on roots is considerable, 

 and it is the deeper-lying younger portions that w^ill be most affected ; the growth of 

 the root itself sets up pressure, and consequent caking of tlie surrounding soil has been 

 recorded. Against such pressure the relatively thick cortex provides a cushion at once 

 efficient and flexible. The xylem, by its peripheral position in the vascular cylinder, and 

 by its centripetal development, is placed as near the absorptive system as is consistent 

 with safety; at the same time the phloem-strands obtain additional protection from 

 their sunken position in grooves between the xylem- groups. As regards avoidance of 

 pressure on the phloem, the advantage afforded by an arrangement in which the phloem 

 is hidden between plates of woody tissue, as compared with the disposal of the phloem in 

 a sheath round the xylem, is well illustrated in the structure of many climbing stems, 

 where, as in the case of roots, the risks arising from lateral pressure are serious ; thus, for 

 example, in several AscLepiadacea^ and Apocynaceae the phloem-strands are sheltered in 

 peripheral grooves of the wood. 



This advantageous arrangement is seen even in some of the abnormal roots of Lycopo- 

 dium and of Ophioglossum, where the xylem takes the form of a crescent enclosing a 

 phloem-group ; but it is most apparent in polyarch roots. 



Simultaneously, this arrangement safely and inexpensively meets strains due to 

 tension ; for there is a concentration of the more resistant tissues into a central strand ; 

 thus also the centripetal development of the xylem affords the further advantage of 

 adding more woody tissue as greater demands are made on the young root. This 

 tendency to concentration of woody tissue is seen in the rarity of a parenchymatous 

 pith : either the xylem-plates approximate towards the centre, or the conjunctive tissue 

 is largely a sclerenchymatous matrix for the vascular strands. 



Such is the scheme of construction in a normal root unaffected by secondary clianges, 

 which attains a compromise between such incompatible conditions as exposure of surface 

 with concentration ; slenderness, flexibility, and delicacy with strength ; together with 

 due prominence of structures requiring shelter. 



In plants having a secondary thickening, the function of mechanical support is under- 

 taken by fit increase in older parts which no longer require to perform the duty of 

 insinuation. 



Loss of liquid in transport is hindered by the relative decrease of surface consequent 

 on increased diameter, and further by the development of cork, the duty of absorption 

 now^ devolving on younger parts of the root. The concomitant loss of facilities for 

 respiration is compensated by the formation of pneumatodes, such as lenticels. 



