450 Minnesota Plant Life. 



Ribbon-shaped leaves are not so common, but are found as 

 the submerged variety in those arrowheads which have two 

 sorts of leaves, and in some of the pondweeds, as well as in 

 the ordinary eel-grass. More frequent are small, numerous, 

 narrow leaves, or leaves cut up into small and narrow divisions. 

 A number of the pondweeds have such submerged leaves, as 

 also the water milfoils, the waterweed, the mare's-tail and the 

 water starworts. Least usual is the quill-shaped leaf, found 

 however, among Minnesota species in the quillworts and in the 

 water lobelias. Some of the pondweeds have broad, thin, 

 papery leaves, crisp like lettuce, in some varieties, but of a 

 texture different from that of land forms. In general, sub- 

 merged leaves have no air pores, or these are \-ery sparingly 

 produced. The epidermis or skin of the leaf is not as strongly 

 developed as it is in the case of terrestrial leaves that are 

 fitted for evaporation. Such leaves are decidedly more absori> 

 tive than are those of land plants, this function being favored 

 by the thin walls of the skin cells. In originating the forms 

 of aquatic leaves the illumination had probably something to 

 do, and the elongated, ribbon-like leaves on submerged por- 

 tions of arrowhead plants may be considered, perhaps, to be 

 extended on account of the semi-darkness caused by the water. 



The stems of these water plants are as various in structure 

 as the leaves. Sometimes, especially where the soil is loose, 

 the stem becomes a creeping, branched rootstock. This may 

 be embedded in the soil at the bottom, as in pondweeds and 

 water-lilies, or it may creep along the bottom, as in water but- 

 tercups and water milfoils. Sometimes the stem is short with 

 the leaves arranged in rosette fashion — for example, the water 

 eel-grass and the quillwort. In all of these plants the stem is 

 perennial. A few annual water ])lants, however, exist, such as 

 the naiads. In such, the stems die when the water freezes and 

 the plant comes up from the seed the following spring. By 

 far the great majority of water plants are perennial. The mass- 

 ive storage organs of bulbous land plants such as the jack-in- 

 the-pulpit, or the onion, are not typical of aquatic vege- 

 tation. In general their stems are characterized by a consid- 

 erable development of the cortical region, together with a poor 

 develoimicnt of the vascular region. ?^Iechanical tissue is not 



