The Garden Magazine, June, 1922 



251 



become converted into a programme of existence for the Cactus 

 and other plants which live in places with alternating periods 

 of drought and rainfall, collecting water from the moist soil 

 during the wet season and using it up during the dry sea- 

 sons, and some of the more massive plants like the great 

 Echinocactus of the Southwest and of Mexico store up a 

 supply which would enable them to grow for two or three 

 years and to survive several more if no additional supply 

 were received. Animals and man have learned to make use 

 of the watery liquid in the soft flesh of some of 

 these plants, as may be seen in photograph on 

 opposite page showing a prospector drinking the 

 sap of the Barrel Cactus of Arizona. Some of 

 the massive succulents like the great Sahuaro 

 (Carnegiea gigantea) form massive trunks in 

 which tons of water may be stored (shown on 

 opposite page). A plant with such a large re- 

 serve of food and water as the Sahuaro may be 

 expected to do some unusual things. Of these 

 none is more interesting than that pictured 

 opposite, in which the main stem of the plant 

 being dead, the skin and pulp have rotted away 

 and the dried skeleton supports a green turgid 

 living branch which may survive for a year or 

 two and form flowers and mature fruit. 



That spines and thorns do form a partial protection to a 

 plant is undeniable. It must be remembered, however, that 

 while the plant is following its own evolution, dozens of animals 

 are likewise developing effectiveness in getting past the offen- 

 sive features and defenses of the plant so that no plant, no 

 matter to what length it has gone, is altogether free from the 

 ravages of animals. That the continuance of these ravages 

 causes the plant to arm itself still more is not supported by an 

 atom of proof. It is not even clear that the best armed Cactus 



BUT we are not through with the story of the 

 sugars and the part they may play in the 

 development of desert plants. So far attention 

 has been paid only to plants which form swollen 

 stems, roots or leaves. Most of these plants 

 have greatly restricted leaf surfaces, and their 

 branches are restricted or done away with en- 

 tirely. This restriction of the development of 

 the branches makes for the spinose type of 

 shrubs and plants of the desert, and when we 

 seek the means and methods by which restric- 

 tion is brought about we again find some perti- 

 nent changes in the sugars, particularly in the 

 mucilages or pentosans which result from the 

 action of drought on 6-carbon sugars. Some 

 of the hexose sugar is more or less constantly 

 passing into mucilages which are an important 

 component of the living matter, and at the 

 same time a much larger proportion is being converted into 

 cell-walls as cellulose or wood, a change which also consists 

 chiefly in the abstraction of water from the molecules of 

 sugar. The anhydride in this case has only a limited ca- 

 pacity for absorbing water and swells but little. Under the 

 influence of aridity the plant makes fewer cells but builds 

 additional material into the walls, making stubby and spiny 

 branches, cylindrical leaves and other organs with restricted 

 surfaces which are so characteristic of the desert. Some of 

 this reaction may be seen when almost any plant is grown 

 in a dry place with insufficient water. The spinose plants 

 have adopted it as a racial or species programme long ago. 

 The Cactus represent in many ways the extreme or highest 

 stage of development of vegetation away from the ancient 

 species which inhabited primeval swamps and water courses. 

 They have the capacity for converting some of their 6-carbon 

 sugars into mucilages which fill huge cells in their monstrous 

 bodies, and at the same time another stream of sugar may be 

 imagined as diverted to the formation of cellulose in walls of 

 external cells and in the construction of their formidable spines. 

 The reader who has given some thought to the armature 

 of plants may feel some disappointment that the view that 

 spines have been developed as protective devices is not given 

 more weight in this discussion. Spines and thorns can be in- 

 creased, lessened, or inhibited by controlling the water supply 

 and humidity in some plants, and they are formed by the con- 

 version of sugars in the manner described. 



SPINELESS PRICKLY PEAR 



Common on the cattle ranges of southern Arizona, the Spineless Prickly Pear 

 (Opuntia santa rita) is being increasingly developed and has distinct possible yalue 

 as an emergency supply for stock on the ranges during a season of restricted rainfall 



survive, as there are a score of spineless Opuntias known in 

 Mexico and three live in the vicinity of the Desert Laboratory 

 (See photograph above). 



THE possibilities of additional food-supplies in plants which 

 contained large proportions of mucilages or derivatives of 

 the 5-carbon sugars attracted serious attention during the Great 

 War. Although many species of seaweed which are high in 

 such substances are consumed in great quantities, especially by 

 the Japanese people, it has not been demonstrated that they 

 are digested to any important extent. The value of all of these 

 plants lies in other components. The slab-like joints of some 

 prickly pears are peeled, sliced, and cooked in fat by the Indians 

 of southern Mexico, but it is not known that any food values are 

 realized beyond those of the minute quantities of the sweet sug- 

 ars present. The mucilages or derivatives of these sugars are so 

 far as known but little changed by the digestive ferments of man. 

 That much use is made of them by cattle and horses seems 

 fairly well established, and the effort which has been made dur- 

 ing recent years to develop Opuntias, some of which are spine- 

 less, has been well warranted by their possible value as a reserve 

 or emergency supply for the stock on the ranges during a season 

 of restricted rainfall. Much has been written as to processes 

 by which wood could be converted into alcohol and food. The 

 mucilages or gums, derivatives of the 5-carbon sugars, univer- 

 sally present in plants, very abundant in many species, constitute 

 a much more promising subject for industrial research. 



