^.86 LECTURE XXIX. 



and dorsi-ventral growth. These apparently purely geometrical relations deter- 

 mine, like polarity, in every respect the external form and internal capacity for 

 reaction of the organ towards external influences. The entire riiode of life of 

 a plant or of a single organ is essentially dependent upon whether it belongs to the 

 radial, bilateral, or dorsi-ventral type. Above all it depends upon these properties 

 in which direction an organ attains its normal position. 



The radial structure is most distinctly shown on transverse sections of roots 

 and perfectly upright shoot-axes. In the transverse section of such an organ the 

 various masses of tissue appear arranged in three, four, or more directions in such 

 a way that if we suppose the transverse section cut in half, the one half is, so to 

 speak, the reflected image of the other, and three, four, or more such divisions may be 

 imagined. If a radially constructed organ branches, the branches which resemble one 

 another appear on it in three, four, or more directions. Particularly clear examples 

 are afforded by shoots with the leaves in whorls; but those with spirally arranged 

 scattered leaves also belong here. The radial structure of an organ is evinced in 

 general as much in its anatomical structure as in its capacity for producing out- 

 growths (roots, leaves, lateral shoots) ; but perhaps the 

 fact is still more important, and at any rate it stands in 

 the closest connection with the above, that radially 

 constructed organs are also sensitive in an equal 

 degree on all sides of the axis of growth to external in- 

 fluences or stimuli ; or, as I have designated it shortly, 

 such organs are orthotropic — i.e. when they are sub- 

 mitted to a directive force acting from without, as 

 '^ gravitation or a ray of light, they become curved 



FIG. 322. -Very young flower otÄ/!^;»« uutll thc axls of growth of the orgau has acquired 



Rhubarb) seen from above : it still consists _ . 



entirely of embryonal tissue. A three of the exactly thc dircctiou ot thc actmg lorce. Mcanwhile 



six perianth leaves; a a the anthers ar- _ _ .1. i- • -i L 



ranged in two whorls ;y the ovary (highly thc objcct hcrc IS ouly to mdicatc prclimmarily the 



magnified). . - , . -r-' 1 



miportance of this property. ror the moment it 

 will be well to render the reader somewhat better 

 acquainted, by means of a few easily intelligible examples, with the meaning of the 

 word Radial. 



Since the expression radial (and the same is true of bilateral and dorsi-ventral) 

 refers to the arrangement of the relations of organisation around the longitudinal axis 

 of an organ or of an entire plant, the relations in question are observed most con- 

 veniently on a transverse section. With respect to the out-shoots from a common 

 axis, the mere view from above, as in Fig. 322, often sufiices for obtaining a satisfactory 

 picture of the arrangement. That this flower, still in an early stage of development 

 and composed essentially of embryonic tissue, is radially constructed is obvious at 

 once if three straight lines are drawn from the central point of the ovary/" to the 

 three outer floral envelopes h, and then produced backwards. The whole flower is 

 divided by each of these three lines into two practically symmetrical halves in each 

 case, and it is unimportant which of these three bisections is taken into consideration ; 

 the halves produced in each case are alike in nature. The radial structure with respect 

 to out-growths also appears very clear on transverse sections of the buds of shoots, as 

 in Fig. 323. If we suppose straight lines to be drawn from its centre, where the 



