A natomico-physio logical Relations in the Spermophyte Shoot. 165 
11. The length of axis traversed by the separate leaf-trace, though 
constant within narrow limits for the species, bears no direct relation to 
the limits of the leaf area laid down in the superficial tissue of the axis. 
Hence it may be said that the delimitation of an inner boundary to this 
region of the leaf is a question solely of definition , but that on the outer 
surface this boundary, in many types, has a real existence capable of 
demoiistra tion . 
12. Two forms of the surface pattern are met with, having the relation 
of mirror images, the individual or axis exhibiting the one or the other 
according as its general orientation is from right to left or from left to right. 
13. If the leaf-insertion width is such that the insertions of all the 
leaves in one cycle of the spiral are together just equal to the circumference 
of the stem (e. g. f in a § spiral or | in a | spiral), the demarcations of 
the extensions will appear as continuous lines along the length of the axis. 
14. In the case of flowering stems the leaf-skin is formed by the bracts 
(when present) and the outermost sepals. 
15. The demarcation of leaf and cotyledon extensions by lines of hairs 
only occurs (so far as observed) when the basal free margins of leaves and 
cotyledons are furnished with hairs. 
16. Demarcation of the boundaries of the leaf-extensions by lines of 
hairs is a character showing what may be termed conditional inheritance, in 
contradistinction to the type of inheritance which is (approximately) constant 
under all conditions. 
17. A further fluctuating development of hairs in response to altered 
conditions may result in a general flooding of the surface, thus blurring the 
effect of the hereditary component. A similar secondary effect may be 
observed under appropriate conditions when the contours are traced out in 
anthocyanin. 
18. The occurrence of adventitious exogenously formed buds on the 
hypocotyl (as e. g. in Antirrhinum Orontium) appears to show that a shoot 
can be formed directly from foliar tissue at undifferentiated points without 
previous mutilation or other injury. 
19. Such buds may show a general flooding with anthocyanin in 
response to the different conditions existing in the embryonic tissue, while 
the parent axis has the colouring matter localized in a definite region. 
In conclusion I wish to express my grateful thanks to Miss Margaret 
Willis, who made the drawings here reproduced. I have also to thank 
Herr W. Engelmann of Leipzig for his courtesy in allowing me to reproduce 
the two figures taken from Sachs’s ‘ Lehrbuch’ and the ‘ Vorlesungen ’. 
The expenses incurred in connexion with this work have been defrayed 
in part by grants from the Royal Society and from the British Association 
for the Advancement of Science. 
