ECOLOGICAL GROUPS 487 



rain. Since there are no living leaves at the base of the flower 

 stalk it has the curious appearance of a blooming stem stuck 

 upright in dry earth. 



Absolutely leafless plants (8) are not very numerous, but 

 there are plants, like the common asparagus and most cacti, 

 which have very small and short-lived leaves that are often 

 nearly or quite functionless. In the common garden asparagus 

 these may be seen as triangular scales on the fleshy shoots 

 sent up in early spring. The green, bristle-like growths on 

 the main branches of asparagus plants throughout the summer 

 perform the offices of leaves but are stem-like in their origin. 

 In the cacti the leaves often appear as awl-shaped, or rather 

 stout, bristle-like organs, borne at the nodes, which soon 

 wither and fall. In such plants the photosynthetic work all 

 the year round is done by chlorophyll-bearing cells close under 

 the epidermis of the stem. In some switch plants a crop of 

 small leaves, borne only during two or three months of early 

 spring, perform active photosynthesis while they last. 



446. Characteristics of mesophytes. Since mesophytes do 

 not live under such conditions as frequently to run the risk 

 either of drowning or of drying up, they do not, as a rule, 

 show extraordinary modifications of structure, such as would 

 enable them to carry on exchange of gases under water or 

 to prevent excessive transpiration in dangerously dry air. A 

 large part of what has been said in the preceding chapters 

 about the structure and. functions of seed plants has had 

 reference to mesophytes, the average plants, and it is 

 therefore unnecessary in this place to go into details in regard 

 to their characteristics. In many respects these are midway 

 between the characteristics of water plants and those of ex- 

 treme xerophytes. In order to fix his knowledge of the sub- 

 ject, the student, after doing what laboratory and field work 

 he can upon the ecological groups discussed in Sects. 440- 

 446, should summarize his impressions by comparing three 

 or more plants, each typical of one of the groups, in tabular 

 form somewhat as follows : 



