Part 14 (1/2)

_In the absence of transverse conduction_, the curvature remains positive (_e.g._, leaflet of _Erythrina_).

_d._ Anisotropic thin organ with high transverse conductivity. Sequence of curvature: transient positive, quickly masked by predominant negative. Light striking on the more excitable side will give rise only to _positive_. The response in relation to the plant, will apparently be in the same direction whether light strikes the organ on one side or the opposite (_e.g._, leaflets of _Mimosa_, _Averrhoa_ and _Biophytum_).

I have shown that tissues in sub-tonic condition exhibit an acceleration of the rate of growth under stimulus (p. 224) the corresponding tropic reaction would therefore be away from stimulus or _negative_ curvature.

The tonic condition is, however, raised to the normal by the action of stimulus itself, and the tropic curvature becomes positive.

I give below a table which will show at a glance all possible variations of phototropic reaction.

TABLE x.x.xI.--MECHANICAL RESPONSE OF PULVINATED AND GROWING ORGANS UNDER LIGHT.

+--------------------------------------------------------------------+

Description of

Action.

Effect observed.

tissue.

+---------------+------------------------+---------------------------+

I Tissue

Stimulus causes increase

Expansion or enhanced rate

sub-tonic.

of internal energy.

of growth, _e.g._,

_Pileus_ of _Coprinus_

drooping in darkness,

made re-turgid by light.

Renewed growth of dark

rigored plant exposed to

light.

II Normally

A 1. Moderate light,

1. Curvature towards light,

excitable

causing excitatory

_e.g._, flower bud of

organ under

contraction of proximal

_Crinum_.

unilateral

and positive expansion

light.

of distal.

A. Organ

A 2. Strong light.

2. Neutralisations, _e.g._,

radial.

Excitatory effect

seedling of _Setaria_.

transmitted to distal,

neutralising first.

A 3. Intense and

3. Reversed or negative

long-continued light.

response, _e.g._, seedling

Fatigue of proximal and

of _Zea Mays_.

excitatory contraction

of distal.

B. Dorsi-

B 1. Excitatory

1. Positive response,

ventral

contraction of proximal

_e.g._, upward folding of

organ.

predominant, owing

leaflets in so-called

either to greater

”diurnal sleep” of

excitability of

_Erythrina indica_ and

proximal or feeble

_c.l.i.toria ternatea_.

transverse conductivity

of tissue.

B 2. Transmission of

2. Negative response,

excitation through