The Power of Movement in Plants, Charles Darwin [the reading list book TXT] 📗

water, had developed secondary or lateral radicles, which were afterwards kept in very damp air, at the proper low temperature for full sensitiveness. They projected, as usual, almost horizontally, with only a slight downward curvature, and retained this position during several days.

Sachs has shown* that these secondary roots are acted on in a peculiar manner by geotropism, so that if displaced they reassume their former sub-horizontal position, and do not bend vertically downwards like the primary radicle. Minute squares of the stiff sanded paper were affixed by means of shellac (but in some instances with thick gum-water) to the tips of 39 secondary radicles of different ages, generally the uppermost ones.

Most of the squares were fixed to the lower sides of the apex, so that if they acted the radicle would bend upwards; but some were fixed laterally, and a few on the upper side. Owing to the extreme tenuity of these radicles, it was very difficult to attach the square to the actual apex.

Whether owing to this or some other circumstance, only nine of the squares induced any

* ‘Arbeiten Bot. Inst., W�rzburg,’ Heft iv. 1874, p. 605-617.

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curvature. The curvature amounted in some cases to about 45o above the horizon, in others to 90o, and then the tip pointed to the zenith. In one instance a distinct upward curvature was observed in 8 h. 15 m., but usually not until 24 h. had elapsed. Although only 9 out of 39 radicles were affected, yet the curvature was so distinct in several of them, that there could be no doubt that the tip is sensitive to slight contact, and that the growing part bends away from the touching object. It is possible that some secondary radicles are more sensitive than others; for Sachs has proved* the interesting fact that each individual secondary radicle possesses its own peculiar constitution.

 

Sensitiveness to contact of the Primary Radicle, a little above the apex, in the Bean (Vicia faba) and Pea (Pisum sativum).—The sensitiveness of the apex of the radicle in the previously described cases, and the consequent curvature of the upper part from the touching object or other source of irritation, is the more remarkable, because Sachs** has shown that pressure at the distance of a few millimeters above the apex causes the radicle to bend, like a tendril, towards the touching object. By fixing pins so that they pressed against the radicles of beans suspended vertically in damp air, we saw this kind of curvature; but rubbing the part with a twig or needle for a few minutes produced no effect. Haberlandt remarks,*** that these radicles in breaking through the seed-coats often rub and press against the ruptured edges, and consequently bend round them. As little squares of the card-like paper affixed with shellac to the tips were highly efficient in causing the radicles to bend away from them, similar pieces (of about 1/20th

 

* ‘Arbeiten Bot. Instit., W�rzburg,’ Heft, iv. 1874, p. 620.

 

** Ibid. Heft iii. 1873, p. 437.

 

*** ‘Die Schutzeinrichtungen der Keimpflanze,’ 1877, p. 25.

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inch square, or rather less) were attached in the same manner to one side of the radicle at a distance of 3 or 4 mm. above the apex. In our first trial on 15 radicles no effect was produced. In a second trial on the same number, three became abruptly curved (but only one strongly) towards the card within 24 h. From these cases we may infer that the pressure from a bit of card affixed with shellac to one side above the apex, is hardly a sufficient irritant; but that it occasionally causes the radicle to bend like a tendril towards this side.

 

We next tried the effect of rubbing several radicles at a distance of 4 mm.

from the apex for a few seconds with lunar caustic (nitrate of silver); and although the radicles had been wiped dry and the stick of caustic was dry, yet the part rubbed was much injured and a slight permanent depression was left. In such cases the opposite side continues to grow, and the radicle necessarily becomes bent towards the injured side. But when a point 4 mm.

from the apex was momentarily touched with dry caustic, it was only faintly discoloured, and no permanent injury was caused. This was shown by several radicles thus treated straightening themselves after one or two days; yet at first they became curved towards the touched side, as if they had been there subjected to slight continued pressure. These cases deserve notice, because when one side of the apex was just touched with caustic, the radicle, as we have seen, curved itself in an opposite direction, that is, away from the touched side.

 

The radicle of the common pea at a point a little above the apex is rather more sensitive to continued pressure than that of the bean, and bends towards the pressed side.* We experimented on a variety (York-

 

* Sachs, ‘Arbeiten Bot. Institut., W�rzburg,’ Heft iii. p. 438.

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shire Hero) which has a much wrinkled tough skin, too large for the included cotyledons; so that out of 30 peas which had been soaked for 24 h.

and allowed to germinate on damp sand, the radicles of three were unable to escape, and were crumpled up in a strange manner within the skin; four other radicles were abruptly bent round the edges of the ruptured skin against which they had pressed. Such abnormalities would probably never, or very rarely, occur with forms developed in a state of nature and subjected to natural selection. One of the four radicles just mentioned in doubling backwards came into contact with the pin by which the pea was fixed to the cork-lid; and now it bent at right angles round the pin, in a direction quite different from that of the first curvature due to contact with the ruptured skin; and it thus afforded a good illustration of the tendril-like sensitiveness of the radicle a little above the apex.

 

Little squares of the card-like paper were next affixed to radicles of the pea at 4 mm. above the apex, in the same manner as with the bean.

Twenty-eight radicles suspended vertically over water were thus treated on different occasions, and 13 of them became curved towards the cards. The greatest degree of curvature amounted to 62o from the perpendicular; but so large an angle was only once formed. On one occasion a slight curvature was perceptible after 5 h. 45 m., and it was generally well-marked after 14 h.

There can therefore be no doubt that with the pea, irritation from a bit of card attached to one side of the radicle above the apex suffices to induce curvature.

 

Squares of card were attached to one side of the tips of 11 radicles within the same jars in which the above trials were made, and five of them became plainly, and one slightly, curved away from this side. Other [page 158]

analogous cases will be immediately described. The fact is here mentioned because it was a striking spectacle, showing the difference in the sensitiveness of the radicle in different parts, to behold in the same jar one set of radicles curved away from the squares on their tips, and another set curved towards the squares attached a little higher up. Moreover, the kind of curvature in the two cases is different. The squares attached above the apex cause the radicle to bend abruptly, the part above and beneath remaining nearly straight; so that here there is little or no transmitted effect. On the other hand, the squares attached to the apex affect the radicle for a length of from about 4 to even 8 mm., inducing in most cases a symmetrical curvature; so that here some influence is transmitted from the apex for this distance along the radicle.

 

Pisum sativum (var. Yorkshire Hero): Sensitiveness of the apex of the Radicle.—Little squares of the same card-like paper were affixed (April 24th) with shellac to one side of the apex of 10 vertically suspended radicles: the temperature of the water in the bottom of the jars was 60o -

61o F. Most of these radicles were acted on in 8 h. 30 m.; and eight of them became in the course of 24 h. conspicuously, and the remaining two slightly, deflected from the perpendicular and from the side bearing the attached squares. Thus all were acted on; but it will suffice to describe two conspicuous cases. In one the terminal portion of the radicle was bent at right angles (A, Fig. 66) after 24h.; and in the other (B) it had by this time become hooked, with the apex pointing to the zenith. The two bits of card here used were .07 inch in length and .04 inch in breadth. Two other radicles, which after 8 h. 30 m. were moderately deflected, became straight again after 24h. Another

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trial was made in the same manner with 15 radicles; but from circumstances, not worth explaining, they were only once and briefly examined after the short

 

Fig. 66. Pisum sativum: deflection produced within 24 hours in the growth of vertically dependent radicles, by little squares of card affixed with shellac to one side of apex: A, bent at right angles; B, hooked.

 

interval of 5 h. 30 m.; and we merely record in our notes “almost all bent slightly from the perpendicular, and away from the squares; the deflection amounting in one or two instances to nearly a rectangle.” These two sets of cases, especially the first one, prove that the apex of the radicle is sensitive to slight contact and that the upper part bends from the touching object. Nevertheless, on June 1st and 4th, 8 other radicles were tried in the same manner at a temperature of 58o - 60o F., and after 24 h. only 1

was decidedly bent from the card, 4 slightly, 2 doubtfully, and 1 not in the least. The amount of curvature was unaccountably small; but all the radicles which were at all bent, were bent away from the cards.

 

We now tried the effects of widely different temperatures on the sensitiveness of these radicles with squares [page 160]

of card attached to their tips. Firstly, 13 peas, most of them having very short and young radicles, were placed in an ice-box, in which the temperature rose during three days from 44o to 47o F. They grew slowly, but 10 out of the 13 became in the course of the three days very slightly curved from the squares; the other 3 were not affected; so that this temperature was too low for any high degree of sensitiveness or for much movement. Jars with 13 other radicles were next placed on a chimney-piece, where they were subjected to a temperature of between 68o and 72o F., and after 24 h., 4 were conspicuously curved from the cards, 2 slightly, and 7

not at all; so that this temperature was rather too high. Lastly 12

radicles were subjected to a temperature varying between 72o and 85o F., and none of them were in the least affected by the squares. The above several trials, especially the first recorded one, indicate that the most favourable temperature for the sensitiveness of the radicle of the pea is about 60o F.

 

The tips of 6 vertically dependent radicles were touched once with dry caustic, in the manner described under Vicia faba. After 24 h. four of them were bent from the side bearing a minute black mark; and the curvature increased in one case after 38 h., and in another case after 48 h., until the terminal part projected almost horizontally. The two remaining radicles were not affected.

 

With radicles of the bean, when extended horizontally in damp air, geotropism always conquered the effects