Page:EB1911 - Volume 13.djvu/245

 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  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.

, in mineralogy, is the mineral commonly called “” (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 (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 (, q.v.) and of symbiotic significance.

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),