Minnesota Plant Life. 465 



leaves of pine trees, the leathery leaves of heaths and hollies, 

 the scale-covered leaves of buffalo-berries and silverberries, the 

 slender cylindrical leaves of many rushes, the ribbon-shaped 

 leaves of many grasses and sedges are all examples of such 

 protective structures. Some leaves are fleshy and succulent, 

 like those of the live-forever, the purslane, the spring-beauty or 

 the portulaca. The leaves of century-plants are types of this 

 kind of adaptation. 



Not only are leaves variously modified, but stem areas as 

 well, in the above-ground portions of xerophytic plants, re- 

 spond to the conditions around them. Thus, some stems are 

 leafless, as in certain of the cacti. Such leafless stems may be 

 somewhat flattened and may have green rinds by which they 

 carry on their starch-making. Sometimes the stems are cylin- 

 drical, as in the bulrush. Sometimes, as in the asparagus, the 

 leaves are absent from the plant body, or are reduced to scales 

 and their place is taken by little needle-shaped branches that 

 carry on the starch-making function. Sometimes the stem is 

 converted into a solid gourd-like body, as in the melon-cacti 

 and prickly-pears. By such forms and structures of leaves and 

 stems the transpiration of moisture is greatly reduced. 



Another modification appears in those plants which have 

 coatings of one sort or another to protect the leaves and stem. 

 Thus, the bloom on the leaves of a century-plant, the hairs and 

 bristles on a mullein leaf, the clusters of dead leaves that are 

 retained by some plants for a year or more, the incrustations of 

 chalk, wax, slime, shellac or gum, all have their value in re- 

 ducing evaporation. The internal sliminess of the leaves of cen- 

 tury-plants is a familiar fact. Well known, also, is the resin- 

 ous matter so commonly seen to exude from the leaves of pines 

 and their allies. Various other adaptations exist which cannot 

 be discussed at all fully. The intimate structures of leaves, 

 the position of the starch-making bodies in the leaves, the 

 position and character of the air pores, all have a significance. 



Some plants have glands the function of which is to secrete 

 salty deposits on the leaves. These salty deposits have a strong 

 affinity for moisture and may serve to collect it from the ex- 

 terior. Many desert plants are known by their pungent odor, 

 very perfect examples of which are the wormwoods or sage- 

 brushes. It has been shown that the vapor of the ethereal oils, 

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