CHAP. III.] PROTECTIVE ARRANGEMENTS. 99 



dered as cactuses by travellers. The object of the above de- 

 scribed modification is obvious ; if ordinary leaves were 

 developed, the extreme heat and aridity would cause tran- 

 spiration of water to take place at a much greater rate from 

 the thin layers of tissue than could be supplied by the 

 roots, the leaves would consequently become flaccid and 

 unable to perform their functions, whereas when the entire 

 mass of tissue is embodied in a solid, succulent trunk 

 furnished with chlorophyll, less surface is exposed, and a 

 phase of life, if not the highest, is maintained. It is a 

 case of doing the best under the circumstances, and, as 

 would be expected, cactuses have not made any startling 

 development since they adopted their present mode of 

 life. As a rule, plants of dry regions are characterized 

 by thick fleshy leaves, and the stem remains green and 

 succulent, thus aiding the leaves in the functions of 

 assimilation, respiration, etc. 



The most modern group of plants, the Angiosperms, 

 are divided into two primary divisions : Monocotyledons, 

 including the palms, lilies, grasses, sedges, etc., and 

 Dicotyledons, that includes all European forest trees 

 except the firs and pines that belong, as previously 

 stated, to the Gymnosperms, and the great bulk of 

 smaller flowering plants excepting the groups named 

 above. The broad features of evolution presented by 

 leaves can be well studied in the sequence presented by 

 the members of our own flora. In Monocotyledons, 

 geologically the oldest group, the leaves are generally 

 long and narrow and with tlie edge or margin entire 

 or uncut, as seen in grasses, sedges, dafi'odils, lily of 

 the valley, etc. Such structures agree literally with 

 the stock definition of a leaf, as given in antiquated 



