October 1, 1896.] 



KNOWLEDGE. 



225 



not lie just inside a phloem group as in the stem, but the 

 two are arranged on alternate radii ; hence such an 

 arrangement of the bundles, which is typical of roots, 

 is known as raclial. Moreover, here the proto-xylem 

 or oldest wood is formed at the outside of tlie primary 

 xylexi. 



It is this position of the proto-xylem with reference to 

 the primary xylem which constitutes the most valuable 

 criterion in distinguishing between sections of stem and 

 root — by no means an easy matter, especially after a deal 

 of secondary thickening has taken place. 



It is clear, that the physiological necessities of the 

 case demand perfect continuity of the corresponding 

 tissues of stem and root, and we will now consider 

 the structure of that part of the axis between the root 

 and cotyledons, viz. : the hypocotyledonary stem or 

 hypocotyl. 



Transition from stem to root. — The work at present done 

 enables us to distinguish at least four good leading types, 

 which we will now proceed to consider in order — taking 

 the simpler modes first : — 



Type I. Ex Common Fumitory {Fumaria officinalis), 

 Pinus, etc. Here the leaf-traces, as they come in from 

 the cotyledons, place themselves in pairs, each pair lying 

 tangentially across the stem with the protoxylems facing 

 one another. The phloem (bast) then goes down into the 

 root unchanged in position, whilst 

 the xylem of each bundle divides 

 into two portions, which diverge and 

 ultimately coalesce with the cor- 

 responding segments of the xylem of 

 its neighbours. Meanwhile the proto- 

 xi/leiiis hare rotated SO as to be, not at 

 the inner limit of the bundle, but at 

 the most peripheral part of the pri- 

 mary xylem of the root. 



In this way we get the transition 

 from the nearly collateral bundles at 

 the upper part to the typically radial 

 arrangement of the bundles at the 

 lower region of the hypocotyl, i.e., 

 where they enter the root. 

 Ex Biota {(.'upressituw), Medicago, etc. Here 



It is the 



Tti'E II. — Diagram 

 showing longitudinal 

 course of the bundles 

 through the I'Vpocotvl. 

 p The pldoem. px Tlie 

 proto-xvlem. 



Type II. 

 •we have the converse case to the foregoing, 

 xylem which goes down unaltered except for the rotation 

 of the proto-xylem, whilst the phloc-m divides in a manner 

 similar to the xylem of Type I. 



Type III. Ex Phaseolus, CheLranthus (wallflower), etc. 



Tyi'K III. — A, Diagram sliowing longitudinal course of the bui'dles 

 through the liypueotvl. V Tlu' pldoi'm. p X Tlie protoxyleiu. 

 B, C, D, Diagraunuatiu transverse sections (four bundles only are 

 shown), li, Through B F. C, Through o ii. D, Koot structure. 



This is a common type where there occur a relatively 

 large number of leaf-traces ; and whereas in Types I. 



and II. there are as many bundles in the root as 

 there are separate groups of xylem and phloem in the 

 upper part of the hypocotyl, here there are only half as 

 many. 



The two xylem strands of each pair fuse, whilst the 

 phloem strands of each pair diverge more and more from 

 each other until ultimately they coalesce with the adjacent 

 phloem strands belonging to the pairs right and left of 

 them. Meanwhile, the proto-xylem has rotated to the 

 periphery of the xylem mass. , 



1)1 the root, therefore, each xylem mass is the direct 

 prolongation downwards of the two xylems of one pair, 

 whilst each phloem mass is the continuation of the phloem 

 of the adjacent bundles belonging to tno pairs. 



If we consider a series of transverse sections — in 

 Phaseoliis multiflnrus — passing through the lower part of 

 the stem, the hypocotyl, and the upper portion of the 

 root, we see the following consecutive changes. At the 

 base of the young stem we have typical collateral leaf- 

 trace bundles. Lower, the cotyledonary bundles enter 

 and join with the leaf-trace bundles, so that immediately 

 below the insertion of the cotyledons we have eight 

 vascular bundles in pairs ; but in each pair the proto- 

 xylem groups are not directed straight inwards, but are 

 turned towards one another. Moreover, the primary 

 xylem is not directly opposite the phloem, but lies so that 

 the two groups of xylem belonging to each pair are nearer 

 to each other than are the corresponding phloem groups. 

 A section still lower will show that the xylem groups 

 approach one another, and ultimately fuse at the same 

 time that their proto-xylem turns more and more towards 

 the outside. 



Meanwhile the phloem strands have undergone a similar 

 change in the reverse direction. A section still lower will 

 show that the xylem has closed up, the proto-xylem is 

 directed straight outwards, and we have reached the typical 

 root structure. 



Type IV. Convolvulus, etc. Here matters are some- 

 what complicated by the structure 

 of the vascular bundles, which are 

 bi-collateral, i.e., each bundle has 

 two groups of phloem, one being 

 situated on the outside of the xylem 

 and a second on its inner side. We 

 have a similar transition to the 

 Phase(dtis type, with the internal 

 phloem superadded. 



The internal phloi-m joins the 

 phloem on the outer side of the 

 bundle, passing in between the 

 converging xylem groups. 



It is evident that this phloem 

 must cross the cambium, but it is 

 not at all clear how this is eft'ected ; 

 whether it passes bodily through 

 (which is improbable), or whether 

 it is continuous through the cam- 

 bium only in a metaphysical sense, 



i.e., in the sense that a medullary ray is continuous 

 through it. 



In all the foregoing types, therefore, in spite of the 

 different arranitenient, the xylem and phloi5m of the main 

 root are the direct downward continuation of the cor- 

 responding tissues of the stem, and this continuity is 

 also preserved in the case of the pericycle as well as 

 the endodermis and other cortical tissues. The oaly 

 aiirupt change is that of the epidermis of the stem to the 

 piliferous layer of the root, /.<■., the layer which produces 

 the root-hairs. 



Typb IV. — Diagram 

 sliowing longitudinal 

 course of the bundles 

 through the hy|x>cotvl. 

 V Kxternal phloem, p p 

 Internal phloem, p i 

 The proto-xylem. 



