The modal number of primary double bundles in the region of transition from root to stem structure at the base of the hypocotyl is four in the dimerous and six in the trimerous and hemitrimerous seedling. In the normal seedlings more than four bundles may occur, but in no case have fewer than this number been observed. In the trimerous seedling variation both above and below the mode is found, the numbers ranging from four to eight. On the average the number is from 1.38 to 1.89 bundles higher (or from 30.5 to 47.0% higher) in the trimerous than in the dimerous seedlings.

Intercalary bundles, which are rather uncommon in seedling anatomy in general, occur in from 11 to 46% of the normal seedlings, whereas they are found in only 9 to 29% of the trimerous and in 28 to 43% of hemitrimerous seedlings. The average number of intercalary bundles is also generally higher in the dimerous plantlets.

Considering the total bundle number at the base of the hypocotyl (primary bundles plus intercalary bundles) the trimerous and hemitrimerous seedlings have from 0.77 to 1.91 bundles, or from 14.4 to 46.7%, more than the dimerous seedlings. The differentiation of the dimerous

and trimerous seedlings is conspicuously shown by the frequency distributions of two of the lines shown in diagram 1.

In passing upward from the base of the hypocotyl, each primary bundle pair normally divides into two so that in the central region of the hypocotyl the bundle number is normally twice the number of primary double bundles at the base, plus the intercalary bundles. In many cases the number is somewhat in excess of this, however, showing either that new (intercalary) bundles have appeared or that some of the bundles have become subdivided.

The modal number of bundles in the mid-region of the hypocotyl is eight or ten in dimerous plantlets; in trimerous and hemitrimerous plantlets

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9 10 11 12 13 14 15 16 17 8 9 10 11 12 13 14 15 16

DIAGRAM 1

Percentage frequency distributions of total bundles (primary double bundles counted as two) at the base of the hypocotyl in dimerous and trimerous seedlings of two lines. Abscissae represent bundle numbers, ordinates represent percentage frequencies.

it is twelve. On the average the number is from 1.7 to 3.8 bundles higher (or from 15.7 to 47.9% higher) in the trimerous than in the dimerous seedlings. The differentiation of the two classes of seedlings in their vascular anatomy at the level is clearly shown in diagram 2.

The bundles in the mid-region of the epicotyl show in dimerous plantlets a modal number of twelve, whereas in trimerous seedlings it is fifteen. On the average there are from 2.8 to 3.7, or from 23.0 to 30.2%, more bundles in the epicotyl of the trimerous than in the dimerous seedling.

The form of the frequency distributions for two of the lines is shown in diagram 3. The epicotyl of the hemitrimerous is in essentials of anatomy identical with that of the dimerous seedling.

Not only are there marked differences in the actual number of bundles, but the variability of bundle number changes from region to region of the seedling, and differs in the three seedling types. Whether judged by range, standard deviation or coefficient of variation, the variability of bundle number in the central region of hypocotyl tends to be distinctly higher in the dimerous than in the trimerous plantlets; but in the epicotyl just the reverse is true, the variability of the trimerous plantlets exceeding that of the dimerous. These differences are conspicuous in diagrams 2

Percentage frequency distribution of number of bundles in central region of hypocotyl in dimerous and trimerous seedlings. Abscissae represent bundle numbers, ordinates represent percentage frequencies.

and 3. In the first case it is the dimerous plantlets, in the second case it is the trimerous ones which show the greater variability. Apparently this is due to differences in the number of intercalary bundles in the hypocotyl, and to the extent of division of the bundles in the epicotyl, of the two types of seedlings.

The coefficients of correlation between various bundle systems also differ widely. In both trimerous and dimerous seedlings there is a negative correlation between the number of primary double bundles and the number of intercalary bundles at the base of the hypocotyl. Thus the number of intercalary bundles is smaller in seedlings with larger numbers of primary double bundles and vice versa. This result for seedlings of the

same (external) morphological type is in agreement with those obtained by a comparison of seedlings which are externally dimerous and trimerous, since the latter frequently have a larger number of primary double bundles but a smaller number of intercalary bundles than the former. In both types of seedlings variation in the number of intercalary bundles is the primary factor in determining variation in the total number of bundles at the base of the hypocotyl.

Turning to the problem of the interrelationship of bundle number at different levels in the seedling we find that there is a substantial correla

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12 13 14 15 16 17 18 19 20 21 12 13 14 15 16 17

DIAGRAM 3

18 19 20 21

Percentage frequency distributions of total bundle number in the central region of the epicotyl of dimerous and trimerous seedlings of two lines. Abscissae represent bundle numbers, ordinates represent percentage frequencies.

tion between the numbers of the three classes of bundles-primary double bundles, intercalary bundles, and total bundles-at the base of the hypocotyl and the number of bundles in the central region of the hypocotyl. In the normal seedlings the coefficients average +0.509 for number of primary double bundles and number of hypocotyledonary bundles, +0.629 for intercalary bundles and hypocotyledonary bundles, and +0.813 for total bundles and hypocotyledonary bundles. In the trimerous plants these correlations average +0.381, +0.238 and +0.598, respectively. The correlations for normal plantlets are practically without exception. higher than those for abnormal seedlings.

The correlations between the number of bundles in the hypocotyl (both basal region and central region) on the one hand and the number of

bundles in the central region of the epicotyl on the other differ conspicuously from those found between the numbers of bundles at the two levels in the hypocotyl itself, in that the coefficients are extremely slight. They are positive in some series, but negative in others.

The regression straight lines and the empirical means for number of bundles in the hypocotyl (lower line) and epicotyl (upper line) associated with various numbers of primary double bundles at the base of the hypocotyl in two of the lines is shown in diagram 4. The irregularity of the means is at least in part due to the fact that the frequencies are largely concentrated in the two classes of 4 and 5 primary double bundles. The

Regression of number of bundles in central regions of hypocotyl and of epicotyl on number of primary double bundles at base of hypocotyl. Abscissae represent primary double bundles, ordinates represent mean

number of bundles in hypocotyl and epicotyl.

slope of the line representing change in hypocotyledonary bundles associaated with variation in number of basal bundles is steep, showing a rather close dependence; but the line showing the change in epicotyledonary bundles associated with the same variations in basal bundles is very slight, showing the laxness of the interdependence of bundle number in these two regions.

On the basis of the data available it is impossible to assert that there is any relationship at all between bundle number above the cotyledons and bundle number below them. These results indicate that there is a complete reorganization of the vascular system at the cotyledonary node.