20 OKNAMENTAL CACTI. 



in structure and others are thin, flat, paperlike, and flexible. Again, 

 in some species the spines are entirely absent. Mamillaria (Pis. I 

 and II) and some groups of Echinocactus have all the variations of 

 characters already described, but differ materially in body structure. 

 In them the ribs or angles have entirely disappeared and are repre- 

 sented by rows of tubercles or mammas, each bearing at its summit 

 a cluster of spines. In this group the tubercles are not arranged in 

 longitudinal rows, but are geometrically tesselated over the plant sur- 

 face, so arranged as to form spirals lunning in both directions about 

 the plant. 



A remarkable and interesting feature is the regularity in number 

 with which these spiral rows appear. As a rule they fall into the 

 numbers 5, 8, 13, 21, 34, 55, and possibly higher numbers. For 

 instance, if it is found that there are 13 parallel rows of tubercles 

 running obliquely around the plant in one direction, there will be 

 either 8 or 21 such parallel rows running obliquely around it in the 

 other direction. Whatever the number of rows counted in one direc- 

 tion, the number counted in the opposite direction will be the one 

 either preceding or following it in the series. Exceptions to this rule 

 are rare, and when one is noted the numbers are usually found to be 

 the doubles of two adjacent numbers in the above series, as 10, 16, 

 26, 42, and so on. Another interesting fact is that each number in 

 the series is the sum of the two immediately preceding it. 



Symmetry is the greatest attraction in this group of plants. Mon- 

 strosities are, however, not infrequent in the family, usually assuming 

 a cristate or cockscomb form of growth. These forms are so odd in 

 appearance that they are frequently sought after, and it is not 

 uncommon to find them represented in the collections of amateurs. 

 Their very grotesqueness commends them to the consideration of 

 collectors. 



SINGLE PLANT DISPLAYS. 



Each individual plant has an attraction of its own. (Pis. XI, XII, 

 and XIII.) Whether it be the symmetrical order of its trunk, its 

 color, its versicolored or versiform spines, or a combination of all 

 these features, supplemented in its proper seasons by the production 

 of flowers and fruits, each normal, healthy plant is well worthy of 

 consideration as an individual specimen. Their adaptability is such 

 as to commend them for situations where many other plants could 

 not exist. They do not require frequent repotting and replemshing 

 of soil, and subsist best on a minimum of water, so that if necessarily 

 neglected for a time they do not materially suffer. A single plant is 

 well worth the little trouble required for its keeping. It occupies a 

 very small amount of space in comparison to its weight, which is an 

 advantage in many instances. 



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