Department of Plant Biology, Southern Illinois University, Carbondale, IL 62901, USA.
Ann Bot. 2013 Oct;112(6):1083-8. doi: 10.1093/aob/mct178. Epub 2013 Aug 26.
Long-lived underground populations of mycoheterotrophic gametophytes and attached sporophytes at various developmental stages occur in lycophytes. Young underground sporophytes obtain carbon solely from the gametophyte and establish nutritional independence only after reaching the soil surface, which may take several years. This prolonged period of matrotrophy exceeds that of bryophytes. The foot is massive and provides the lifeline for sporophyte establishment, yet the fine structure of the placental region is unexplored in lycophytes with underground gametophytes.
Gametophytes with attached embryos/young sporophytes of Lycopodium obscurum were collected in nature, processed and examined by light and transmission electron microscopy.
Three ultrastructurally distinct regions were identified within a single foot of a sporophyte emerging from the soil. Young foot regions actively divide, and have direct contact with and show little differentiation from gametophyte cells. In unlobed foot areas, cells in both generations exhibit polarity in content and indicate unidirectional transport of carbon reserves into the foot toward the developing shoot and root. The foot has inconspicuous wall ingrowths. Highly lobed foot regions contain peripheral transfer cells with prominent wall ingrowths that absorb nutrients from degenerating gametophyte cells.
Variability within a single placenta is consistent with an invasive and long-lived foot. The late appearance of wall ingrowths in transfer cells reflects this dynamic ever-growing embryo. Placental features in lycophytes are related to the unique reorientation of all embryonic regions during development. Small placentas with wall ingrowths in both generations characterize ephemeral embryos in green gametophytes, while short-lived and repositioning embryos of heterosporous taxa are devoid of transfer cells. Transfer cell evolution across embryophytes is riddled with homoplasy and reflects diverse patterns of embryology. Scrutiny of placental evolution must include consideration of nutritional status and life history strategies of the gametophyte and young sporophyte.
石松类植物的地下配子体和不同发育阶段的附生孢子体具有长寿的地下种群。年轻的地下孢子体仅从配子体获得碳，并在到达土壤表面后才建立营养独立性，这可能需要数年时间。这种长期的母体营养超过了苔藓植物。足部庞大，为孢子体的建立提供了生命线，但具有地下配子体的石松类植物的胎盘区的精细结构尚未被探索。
在野外收集了具有附生胚胎/年轻孢子体的石松属植物的配子体，通过光镜和透射电子显微镜进行处理和检查。
从土壤中萌发的孢子体的单个足部中鉴定出三个具有不同超微结构的区域。年轻的足部区域积极分裂，与配子体细胞直接接触，几乎没有分化。在无裂片足部区域，两代细胞的内容物都表现出极性，并表明碳储备单向向发育中的芽和根运输到足部。足部没有明显的壁内突。高度裂片的足部区域包含周边转移细胞，具有明显的壁内突，可从退化的配子体细胞中吸收营养。
单个胎盘内的变异性与具有侵袭性和长寿的足部一致。转移细胞中壁内突的出现较晚反映了这种动态的不断生长的胚胎。石松类植物的胎盘特征与发育过程中所有胚胎区域的独特重新定向有关。具有配子体的小胎盘具有壁内突，特征为短暂的绿色配子体胚胎，而具有短暂和重新定位的孢子体的异孢体类群则缺乏转移细胞。整个胚胎植物的转移细胞进化充满了同形现象，反映了胚胎学的多样化模式。胎盘进化的审查必须包括考虑配子体和年轻孢子体的营养状况和生活史策略。
