. The Canadian field-naturalist. Figure 1. Lateral view of Isoetes maritima megaspore (Porsild & Breinmg 11,502, Sheldon Lake, Yukon Territory [GH]; Scale bar = 100 |im). Figure 2. Lateral view of Isoetes echinospora megaspore (Britton 9,523, Parry Sound District, Ontario [OAC]; Scale bar = 100 jam).. Figure 3. Microspores of Isoetes maritima {Porsild & Breitung 11,502, Sheldon Lake, Yukon Territory [S]; Scale bar = 10 pm). Figure 4. Microspore of Isoetes echinospora {Britton 9,523, Parry Sound District, Ontario [OAC]; Scale bar = 10 m). throughout (Taylor et al. 1993; Britton and Brun
. The Canadian field-naturalist. Figure 1. Lateral view of Isoetes maritima megaspore (Porsild & Breinmg 11,502, Sheldon Lake, Yukon Territory [GH]; Scale bar = 100 |im). Figure 2. Lateral view of Isoetes echinospora megaspore (Britton 9,523, Parry Sound District, Ontario [OAC]; Scale bar = 100 jam).. Figure 3. Microspores of Isoetes maritima {Porsild & Breitung 11,502, Sheldon Lake, Yukon Territory [S]; Scale bar = 10 pm). Figure 4. Microspore of Isoetes echinospora {Britton 9,523, Parry Sound District, Ontario [OAC]; Scale bar = 10 m). throughout (Taylor et al. 1993; Britton and Brunton 1996; Britton, Brunton, and Talbot 1999). The slightly flattened proximal hemisphere and swollen distal hemisphere of many /. maritima megaspores also gives them a somewhat "acorn-like" profile, in contrast to the usually round shape of /. echinospora megaspores. The microspores of Porsild and Breitung 11,502 plants are relatively large ( jam, N = 20). Even though they appear to be slightly immature, these microspores exhibit the spine-tipped pauperculate ornamentation indicative of /. maritima (Figure 3). Isoetes echinospora microspores on the other hand (Figure 4), are smooth-surfaced and significantly smaller ( |am - Brunton and Britton 1996; Britton, Brunton, and Talbot 1999). We also examined the single additional collection of Isoetes known from the Yukon; it is from the southeastern portion of the Territory as well (Rosie 1991; Cody 1996). The label data for that collection states: "on mud and sand bottom, shallow water along shore of lake. Hour Lake, near Frances Lake, 61° ir N, 129° 08' W, i?. Rosie 897, 29 September 1979" (DAO!, VIC!). These plants exhibit the dense- ly echinate megaspores covered with long, thin- pointed, narrow spines indicative of /. echinospora. The latter, then, constitutes the only known Yukon TeiTitory population of/, echinospora (Figure 5). Habitat and Site Sheldon Lake is atypical of the spring-fed, clear- wa
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