156 



THE DICTIONARY OF GARDENING, 



Flasmodiophora Brassica continued. 



Microscopic inspection of a thin slice from a diseased 

 portion of a root, shows numerous cells of the tissue of 

 the host-plant hardly altered in any way; but, inter- 

 mixed with these, there are many others, from four to six 

 times their diameter, which are occupied by the parasite ; 

 and it is these enlarged cells that alter the colour of the 

 root, and make its surface in a transverse section, when 

 a little magnified, look mottled. In the newly-diseased 

 roots, these enlarged cells inclose a slimy, yellowish, 

 granular plasm, in which are usually numerous cavities, 

 filled with cell sap or with air. If examined at a later 

 period, these enlarged cells are found filled with myriads 

 of the minute, globular, thin-walled, transparent spores 

 of P. Brassicce. These remain uninjured during the 

 keenest frosts of winter, and form a large part of the 

 powdery material that fills up the decayed root between 

 the woody bundles, and they become widely scattered 

 through the soil by the action of wind and rain. When 

 the weather becomes warmer, in the following spring, they 

 begin to germinate, the cell-wall splits, and the proto- 

 plasmic contents pass out, and creep about through the 

 damp soil like Amoeba!, and also by the help of a slender 

 hair or cilium at one part. When they come into contact 

 with others like themselves, they unite completely, and 

 form larger plasmodia. If they meet with young roots of 

 Cabbages, Turnips, Charlock, or Wild Mustard, they make 

 their way into them, and produce the disease in them also. 



This disease has, of late years, been spreading far and 

 wide, and has proved very destructive in Turnip fields ; 

 so much so, that, in some districts, Turnips are no longer 

 a profitable crop. It also attacks Turnips and Cabbages in 

 vegetable gardens, though the less extended diffusion of 

 the host-plants renders the attacks less fatal, and remedies 

 more readily applicable. Frequent experiments have shown 

 that, in soil in which diseased roots have been allowed to 

 rot, a new crop of Cabbages, or other host-plants, will 

 certainly suffer from disease if sown in that soil in the 

 ensuing year. Even after a year has elapsed, the crop 

 is liable to suffer from this cause, though to a less extent. 

 Yet farmers almost always, and gardeners frequently, 

 leave the diseased roots in the soil, as not repaying the 

 labour of pulling them up. Experiments have been con- 

 ducted by Mr. T. Jamieson, to ascertain the result of 

 different manures on this disease, and have led him to 

 the conclusion that the use of superphosphates, or soluble 

 phosphates, is followed by a great increase of disease ; 

 and that ground coprolites, and ground or steamed 

 bone-flour, are the manures that best enable the 

 plants to resist it ; but these conclusions are opposed by 

 other experimenters. If the effects on Charlock, Wild 

 Mustard, and Cabbages, are compared with those on 

 cultivated Turnips, it is at once seen that the last plant 

 suffers far more severely. The cause seems to be that 

 the Fungus attacks the cellular tissue only, and this in 

 Turnips is very largely developed, as it is this that 

 renders the roots useful for food, and, accordingly, it 

 has been promoted by the selection of those varieties 

 for propagation that produce the largest roots with least 

 woody fibre in them. The former plants are crippled, 

 but are generally able to produce some seeds; but the 

 Turnips are, in general, destroyed before they reach the 

 stage to form seeds. 



Remedies. All diseased roots ought to be collected at 

 as early a stage as possible, and destroyed by burning, 

 if this is practicable. Ground where Turnips have become 

 diseased should not have Turnips or Cabbages grown in 

 it for at least two years, to permit any spores in the 

 soil to germinate, and thus to starve and kill the plas- 

 modia. With the same object, all Charlock and Wild 

 Mustard should be carefully eradicated, both from the 

 ground and from its neighbourhood. Further experiments 

 upon the effect of different manures are desirable, but, in 

 the meantime, they point to ground coprolites, and to 



Flasm.odioph.ora Brassicae continued. 

 ground or steamed bone-flour, as the best artificial 

 manure for Turnips. Unfortunately, no means are 

 known for the cure of plants already attacked, pre- 

 vention alone being practicable in any way. 



PLATANACE2E. A very small natural order of 

 usually tall trees, with flaking bark ; they inhabit the 

 temperate or sub-tropical regions of the Northern hemi- 

 sphere, two are natives of East Europe or Asia, and the 

 rest are American. Flowers monoecious, in unisexual, 

 globose, densely-crowded heads ; " the stamens in the 

 males, and the ovaries in the females, are mixed, with- 

 out definite order, with scales, which may be bracts, 

 perianth segments, or staminodes, or arrested ovaries" 

 (J. D. Hooker). Fruit a nut. Leaves alternate, petio- 

 late, broad, palmately nerved and lobed ; petioles dilated 

 at base ; stipules membranous, caducous. The only genus 

 of the order Platanus comprises only five or six species, 

 which are mainly valuable for their timber and orna- 

 mental appearance. 



PLATANTHERA. Included under Habenaria 

 (which see). 



PLATANUS (Platanos, the old Greek name, from 

 platys, broad ; referring to the foliage). Plane-tree. The 

 only genus of OBD. Platanaceaz (which see for characters, 

 &c.). The two species describe 1 below are magnificent 

 trees for parks and similar situations. Both require a 

 deep, rich, soft soil, and generally attain the greatest 

 size where their roots have access to water. They 

 require shelter, but must not be confined. Propagated 

 by seeds, or by layers. The former are contained in 

 round balls, which require to be broken, and should be 

 sown in March ; merely pressing them into the surface of 

 the soil is sufficient, but they must be kept moist and 

 shaded. The quickest way to propagate is from layers. 

 P. occidentalis (Western).* Button- wood; Western Plane. 

 fl. greenish. May. fr. brownish ; ripe in October and November. 

 I. five-angled, obsoletely lobed, dentate, wedge-shaped at the 

 base, downy beneath, h. 70ft. to 80ft. Atlantic and Western 

 States, 1636. A fine species, differing from P. orientalis in its 

 less deeply-lobed, more coriaceous, pubescent leaves, and in the 

 fertile catkins being solitary on the long peduncles. It is rare in 

 ot so hardy as the common 



British gardens, and apparently n 

 species. (E. T. S. M. ed. 2, 261.) 



P. orientalis (Eastern).* Oriental or Common Plane, fl. greenish- 

 yellow. April, fr. brown ; ripe in October, and persistent for 

 the greater part of the winter. I. five-lobed, palmate, wed^e- 

 shaped at the base, the divisions lanceolate, sinuated ; stipules 

 nearly entire. h. 60ft. to 80ft. Levant, previous to 1548. 

 (W. D. B. 101.) A beautiful tree, presenting a great variety of 

 handsome forms, which differ chiefly in the shape and lobing of 

 the leaves. The variety acerifolia (Maple-leaved) is the com- 

 monest in cultivation, frequently bearing the name of P. occi- 

 dentalis, from which it may be readily distinguished when in 

 fruit by the peduncles bearing more than one ball, and 

 frequently many. It is the form known as the London Plane, 

 on account of its being generally planted in the parks ; and is 

 erect-growing tree, with usually three-lobed leaves, or, if five- 



lobed, less deeply so than in the typical form. (W. D. B. 100, 

 under name of P. occidentalis.) The typical orientate is a more 

 spreading tree, with very large, deeply nve-lobed leaves, cordate 



or truncate at the base." The variety 'euneata has the leaves dis- 

 tinctly wedge-shaped at the base ; laciniata, very deeply, much- 

 divided leaves ; and variegata, variegated foliage. 

 A plant in gardens, with fine bold foliage, called P. californica, 

 has not yet fruited in this country ; it is doubtful whether it really 

 is the Californian P. racemosa (with which the true P. cali- 

 fornica is synonymous). 



PLATYCAPNOS. Included under Fumaria (which 

 see). 



PLATYCARPHA (from platys, broad, and carphe, 

 chaff ; in allusion to the broad, chaffy scales of the in- 

 volucre). OBD. Composites. A genus consisting of a 

 couple of species of stemless perennials from South 

 Africa. Flower-heads purple, densely crowded, many- 

 flowered, homogamous, sessile. Leaves numerous, stalked, 

 spreading like a star upon the ground, pinnately divided, 

 with coarsely-toothed lobes and pungent teeth. P. 

 glomerata is a pretty and interesting plant; it thrives 

 in a well- drained, sandy soil. 



