Part 15 (1/2)

Lenticels are also formed on some leaf-galls, and are remarkable as being structures not normal on leaves.

_Pustules._--This term may be employed generally for all slight upheavals of the surfaces of herbaceous organs, which subsequently burst and give egress to the spores, etc., of the organism causing them, or merely fray away at the top if no organism is discoverable. They are often due to fungi--_e.g._ _Synchytrium_, _Protomyces_, _Cystopus_, and Ustilagineae,--and we may extend the use of the general term also to those cases where the _stroma_ of the fungus itself bursts through the cortex of older parts and forms the princ.i.p.al part of the pustule--_e.g._ _Monilia_, forming white or grey pustules on Apples, _Roestelia_ and other aecidia, forming yellow or orange pustules on leaves, etc.; _Cucurbitaria_ and _Nectria_ (red) breaking through the cortex of trees, and _Phoma_ and numerous other Ascomycetes which form black cus.h.i.+ons. _Pustules_ on the leaves of _Lysimachia_, _Ajuga_, etc., are due to the parasitic Alga _Phyllobium_.

Cylindrical stem swellings are caused by _Calyptospora_: they are due to the hypertrophy of the cortex of Bilberry stems permeated by the hyphae. _Epichloe_, which clothes the sheaths and halms of gra.s.ses with its stroma, at first snowy white and later ochre-yellow as the perithecia form, is another example.

The cylindrical layer of eggs of a moth such as _Bombyx_ on a twig must not be confounded with these cases.

_Frost-blisters_ are pustule-like uprisings of the cortex, where the living tissues below have formed a callus-like cus.h.i.+on into the cavity beneath the dead outer parts of the cortex which were killed by the frost; they occur on the stems of young Apples, Pears, etc.

_Galls_ in the narrower sense are tissue outgrowths usually involving deeper cell-layers. They are so varied and numerous that cla.s.sification is difficult. For symptomatic purposes we may divide them as follows:

_Leaf-galls._--A well-marked type is that of the _pocket-galls_ or _bladders_ in which the whole thickness of the leaf is as it were pushed up like a glove-finger at one spot, so that if the upper surface of the leaf forms the outside of the gall the lower surface is its lining. Such galls are common on Limes (_Phytoptus_), _Glechoma_ (_Cecidomyia_), Elms (_Tetraneura_), etc. Similar localised extension of the leaf surface, compelling it to rise up like a pocket, are caused by fungi--_e.g._ _Taphrina_ on Poplars, _Exoascus_ on Birches, etc., _Exobasidium_ on Bilberries, Rhododendrons, etc.

Another type is that of the _Gall-apple_, so well known on Oaks, where the spherical swelling is solid--except for the inner cavity containing the eggs--_Neurotus_, _Cynips_, _Hormomyia_, etc. These are comparable in general characters to the nodules on roots.

Fungus galls with similar external features when young are found on Maize (_Ustilago Maydis_), and betray their nature by the black powdery spores as they mature.

Bud galls on Willows are due to _Cecidomyia_, which causes several internodes to swell out into a greenish barrel-shaped ma.s.s, from which leaves may spring.

Small irregular excrescences on Willow stems are referred to _Phytoptus_, and another species of the same insect induces similar swellings on Pines which are not surcharged with resin.

_American Blight_, or Woolly Aphis, on Apples especially, causes the tumour-like swellings covered with sticky white fluff, which is a waxy excretion of the insect. Galls on _Pilea_, in Java, are due to an Alga--_Phytophysa_.

_Root-nodules_ or _nodosities_ are frequently caused by insects--_e.g._ _Centhorhynchus_, a beetle which attacks Crucifers, _Cynips_ and allied ”gallflies” of Oaks, and the notorious _Phylloxera_. But similar root-galls are produced by Nematode worms, _Heterodora_, on Beets, Tomatoes, Cuc.u.mbers and numerous other plants, and by the Slime fungus _Plasmodiophora_, and it is not always easy to distinguish such cases from the fungus-galls (_Mycocecidia_) on the roots of Alders, _Juncus_, and Leguminoseae where the symbiosis of bacteria or fungi with the roots are of benefit to the plant. _Urocystis Leimbachii_ forms similar nodules at the collar of young plants of _Adonis_.

_Heterodora javanica_ pa.s.ses into the cortex of sugar-cane roots through fissures, and makes its way to the place where a young rootlet is about to emerge; here it sticks its beak into the growing-point and remains fixed.

Molliard has shown that in the roots of Melons, _Coleus_, etc., _Heterodora_ causes the cells in immediate contact with its head, and which would normally become vessels of the xylem, to swell up into huge giant-cells, with their walls curiously folded, and containing large supplies of proteids and numerous nuclei, reminding us of the food-layer of insect galls and of the tapetal layer of pollen-sacs. While the stimulus exerted by the Nematode thus induces hypertrophy and storage with food-substances of these cells, those of the next layers undergo reticulate thickenings of their walls. Again instances of the evolution of new tissue elements by the action of the foreign organism.

So far as galls on leaves are concerned the amount and kind of damage done are in proportion to the area of chlorophyll action put out of play for the benefit of the plant, and the remarks already made on p. 193 apply here also. Where buds are destroyed the effects may of course extend further, but it rarely happens that leaf-galls are so abundant as to maim a tree permanently. Nevertheless we must remember that cases like _Phylloxera_ are notorious.

Far more dangerous, however, are the root-galls due to such insects, because here the damage is not so local: the water-supplies are cut off, and injurious consequences result from the absorption of the products of decomposition in the soil.

NOTES TO CHAPTER XXIII.

In addition to the literature on galls quoted in the Notes to Chapter XIV., the reader should consult Dale ”On certain Outgrowths (Intumescences) on the green parts of Hibiscus,”

_Proc. Cambr. Phil. Soc._, Vol. X., 1899, p. 192, and _Brit.

a.s.s. Rep._, Bradford, 1900.

The detailed study of the anatomy and histology of Galls has been recently undertaken by Kuster, ”_Beitrage zur Kenntniss der Gallenanatomie_,” Flora, B. 87, 1900, p. 117, where the princ.i.p.al references will be found.

On the root-galls due to Nematodes see Atkinson in _Science Contributions from the Agric. Expt. Station, Alabama_, Vol.

I., p. 1, 1889; Percival, ”An Eel-worm disease of Hops” in _Natural Science_, Vol. VI., 1895, p. 187; and Molliard in _Revue generale de Botanique_, Apl., 1900, p. 157, where the histology is dealt with.

The nodules of the roots of Leguminoseae are not part of the subject of this work: the literature is collected in _Science Progress_, 1895, Vol. III., p. 252, and Dawson, _Phil.