ИСТИНА

Hydrorhiza (system of stolons) is a polyfunctional organ in colonial hydroids. Three functions are restricted to stolons: 1) attachment to substratum; 2) ensuring resilience in hostile conditions; 3) formation of new stems or hydranths (in creeping colonies). Four other functions are performed by the coenosarc of both stolons and stems: 4) branching (formation of lateral branches of stolons and stems); 5) longitudinal terminal growth; 6) food digestion (parietal digestion); 7) cell division. Stolons are usually characterized by simple and monotonous morphology, which has caused the significance of their spatial structure to be largely overlooked. However, the microanatomy of stolons is hardly monotonous, as demonstrated by tissue studies using standard histological methods of both optical and electron microscopy. Our study focused on rectilinear stolons of G. loveni. Colonies were grown on artificial substratum in stable laboratory conditions, with a strict feeding regime using newly hatched Artemia nauplii. The aim of the study was to determine characteristic segments in the stolons differentiated by shape and proportion of the main cell types. Since G. loveni stolons are rectilinear and tube-like, spatial differences between segments can be determined linearly: by distance from the apex of the stolon along its longitudinal axis. The primary histological method consisted of analysis of serial semi-thin 1 μm cross-sections. Perisarc diameter averages 0.1 mm, barely varying along the length of the stolon, while coenosarc diameter inside the perisarc may vary between 0.1 and 0.07 mm. Time-lapse video microscopy was used to record coenosarc pulsations, and transport of cells and hydroplasm. A growing stolon has at least 7 morphofunctional zones: 1) frontal part of the apex or growing tip (GT) (length: L = 0.04-0,06 mm), characterized by growth pulsations and secretion of chitinous perisarc; 2) subfrontal segment of the GT with thickened gastrodermis (L ≈ 0.25 mm) that generates longitudinal pulsations, causing the advancement of the GT inside the perisarc, i.e. growth; 3) proximal segment of the GT with gastrodermis of regular thickness (L ≈ 0.25 mm), where new stems are occasionally formed; 4) subapical part of the coenosarc characterized by significant elasticity and high amplitude of transverse pulsations; its length is limited by the length of the terminal internode of the stolon; these segments are the principal drivers of hydroplasmic flow in the gastric cavity of the colony; 5) coenosarc of the first stolonal internode – zone of specialized cell accumulation and transportation; 6) parts of the coenosarc removed from the GT are characterized by a high concentration of i-cells; this is where stolon branching may occur; 7) stolon resorption zone (not yet studied in detail).