10 THE AMERICAN MIDLAND NATURALIST 
part of the annual growth are also at first collateral, but winter 
overtakes these in an undeveloped condition. ‘The following 
spring most of these bundles divide at their growing point, part 
of the new elements going upward into a new aérial stem and 
part into a further extension of the rhizome. On account of the 
great demand for soil sap in the new stem the unfinished col- 
lateral bundles in the adjacent constricted part of last year’s 
rhizome growth develop an unusual amount of xylem in connection 
with new secondary roots and become amphivasal. In the more 
distant expanded part of the rhizome the continuations of these 
bundles being already mature remain collateral as at first, but 
many of them develop excessive numbers of xylem elements which 
do not,. however, encircle the phloem (Fig. 17.) 
EXPLANATION OF THE FIGURES. 
Figs. 1-9. Illustration of the stages of development of the seedling 
from the time of germination until the end of the second year’s growth. 
All of the illustration are exactly life size. Fig. 1. Germinating seed with 
primary root and hypocotyl emerging. Fig. 2. Later stage with elongated 
primary root, swollen hypocotyl, and the slit through which the epicotyl 
is to emerge. Fig. 3. The epicotyl appearing through the slit. -Fig. 4. 
Stage in the further development of the epicotyl and primary root. Figs. 
5 and 6. Formation of secondary roots. Fig. 7. Elongation of epicotyl 
and production of leaf scales. Fig. 8. Seedling with leaf in second season 
of growth. The cotyledon has disappeared and the primary root is withering. 
Fig. 9. A later stage in the second year’s growth showing the formation 
of the second tuber. 
Fig. 10. Cross section of the petiole of the seedling cotyledon. (a) 
epidermis, (b) cortical tissue, (c) phloem, (d) xylem. 
Fig. 10x. Food storage cells of the seed endosperm. 
Fig. roy. Longitudinal diagrammatic section of the tissue systems 
in a young seedling. ‘The side figures show diagrammatic cross sections 
at the places indicated by the dotted lines. (a) petiole of cotyledon, (b) 
hypocotyl, (c) primary root, (d) seed, (e) cotyledon, (b) epicotyl. 
Fig. 11. Cross section of the primary root of the seedling. (a) epiblema, 
(b) palisaded layer of the periblem, (c) ordinary periblem cells, (d) endoder- 
mis, (e) pericycle, (f) phloem and (g) xylem, constituting the plerome, 
(h) raphides. 
Fig. 12. Cross section of the hypocotyl. (a) dermatogen, (b) periblem, 
(c) bundle leading to epicotyl, (d) cambium layer, (e) central parenchy- 
matous tissue, (f) bundle leading to cotyledon. 
Fig. 13. Cross section of the leaf of amature plant. (a) upper epider- 
mis, (b and c) palisaded layer, (d) wood bunble, (e) lower epidermis, (f) 
stoma, (g) loose chlorenchyma. 
Fig. 14. Surface view of upper epidermis showing cell structure. 
