be kept somewhat dry, and in spring the ball of soil should be reduced and the plants repotted, the shoots being slightly pruned, so as to maintain a symmetrical head. When they are planted out against the walls and pillars of the greenhouse or conservatory an abundance of highly perfumed blossoms will be supplied all the year round. From the end of May till October heliotropes are excellent for massing in beds in the open air by themselves or with other plants. Many florists’ varieties of the common heliotrope are known in cultivation.
Pliny (Nat. hist. xxii. 29) distinguishes two kinds of “heliotropium,” the tricoccum, and a somewhat taller plant, the helioscopium; the former, it has been supposed, is Croton tinctorium, and the latter the ἡλιοτρόπιον μικρόν of Dioscorides or Heliotropium europaeum. The helioscopium, according to Pliny, was variously employed in medicine; thus the juice of the leaves with salt served for the removal of warts, whence the term herba verrucaria applied to the plant. What, from the perfume of its flowers, is sometimes called winter heliotrope, is the fragrant butterbur, or sweet-scented coltsfoot, Petasites (Tussilago) fragrans, a perennial Composite plant.
Heliotrope, in mineralogy, is the mineral commonly called “bloodstone” (q.v.), and sometimes termed girasol—a name applied also to fire-opal. The name, like those of many ancient names of minerals, seems to have had a fanciful origin. According to Pliny the stone was so called because when thrown into the water it turned the sun’s light falling upon it into a reflection like that of blood.
HELIOZOA, in zoology, a group of the Sarcodina (q.v.) so
named by E. Haeckel, 1866. They are characterized by the
radiate pseudopods, finely tapering at the apex, springing
abruptly from the superficial protoplasm, containing a denser,
rather permanent axial rod (figs. 1 (1), 2 (2)); protoplasm without
a clear ectoplasm or pellicle, often frothy with large vacuoles,
like the alveoli of Radiolaria; nucleus 1 or numerous; skeleton
absent, gelatinous or of separate siliceous fibres, plates or
spicules, rarely complete and latticed; reproduction by simple
fission or by brood-formation, often syngamous; form usually
nearly spherical, rarely changing slowly. This group was
formerly included with the Rhizopoda; but was separated
from it by Haeckel on account of the character of its pseudopods,
and its general adaptation to a semipelagic existence correlated
with the frothy cytoplasm (fig. 1 (1)). Actinophrys sol and
Actinosphaerium eichhornii (fig. 2), known as sun animalcules
to the older microscopists, float freely in stagnant or slow-flowing
waters, and Myriophrys is able by an investment of
long flagelliform cilia to swim freely. The majority, however,
lurk among confervae or the light débris of the bottom ooze;
and come under the head of “sapropelic” rather than pelagic
organisms. The body is usually of constant spherical form in
relation to the floating habit. Nuclearia, however, shows amoeboid
changes of general outline. The pseudopods are retractile,
the axial filament being absorbed as the filament grows shorter
and thicker and disappearing when the pseudopod merges into the
ectoplasm, to be reformed at the same time with the pseudopod.
There is often a distinction, clear, but never sharp, between the
richly vacuolate, almost frothy ectoplasm and the denser
endoplasm. One or more contractile vacuoles may protrude
from the ectoplasm. The endoplasm contains the nucleus or
nuclei. The nucleus when single may be central or excentric:
in the latter case, the endoplasm contains a clear central sphere
(“centrosome”) on which abut the axial filaments of the pseudopods.
The ectoplasm contains, in some species, constantly
(Raphidiophrys viridis) or occasionally (Actinosphaerium), green
cells belonging to the genera Zoochlorella and Sphaerocystis, both
probably—the latter certainly—vegetative stages of a Chlamydomonad
(Flagellata, q.v.) and of symbiotic significance.
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Fig. 1.—Heliozoa. 1. Actinophrys sol, Ehrb. a, food-particle lying in a large food-vacuole; b, deep-lying finely granular protoplasm; c, axial filament of a pseudopodium extended inwards to the nucleus; d, the central nucleus; e, contractile vacuole; f, superficial much vacuolated protoplasm. 2. Clathrulina elegans, Cienk. 3. Heterophrys marina, H. and L. a, nucleus; b, clearer protoplasm surrounding the nucleus; c, the peculiar felted envelope. 4. Raphidiophrys pallida, F. E. Schultze. a, food-particle; b, contractile vacuole; c, the nucleus; d, central granule in which all the axis-filaments of the pseudopodia meet. The tangentially disposed spicules are seen arranged in masses on the surface. 5. Acanthocystis turfacea, Carter. a, probably the central nucleus; b, clear protoplasm around the nucleus; c, more superficial protoplasm with vacuoles and chlorophyll corpuscles; d, coarser siliceous spicules; e, finer forked siliceous spicules; f, finely granular layer of protoplasm. The long pseudopodia reaching beyond the spicules are not lettered. 6. Bi-flagellate “flagellula” of Acanthocystis aculeata. a, nucleus. 7. Id. of Clathrulina elegans. a, nucleus; b, granules. 8. Astrodisculus ruber, Greeff. a, red-coloured central sphere (? nucleus); b, peripheral homogeneous envelope. |
The Heliozoa can move by rolling over on their extended pseudopods; Acanthocystis ludibunda traversing a path of as much as twenty times its diameter in a minute, according to Penard. Several species (e.g. Raphidiophrys elegans) remain associated by the union of their pseudopods, whether into social aggregates (due to approximation) or “colonies” due to lack of separation after fission, is not accurately known. The multinuclear species Actinosphaerium eichhornii (fig. 2), normally apocytial (i.e. the nuclei divide repeatedly without division of the cytoplasm),
