16 



GARDENERS' CHROMCLE 



Desert Vegetation 



WILLARD N. CLUTE 



II'" you leave it to the dictionary, n desert is a region 

 alnn)st destitute of moisture and vegetation but this 



definition does not entirely satisfy the botanist. Me 

 knows of various regions on the earth where, notwith- 

 standing an abundance of water, plants are few and far 

 between, and on the other hand, he is familiar with the 

 fact that many other areas, regarded as deserts, may 

 be veritable flower-gardens for pari of each year. In 

 the Arctic regions and on mountain tops, for instance, 

 there is plenty of water but in a form which plants can- 

 not use. There are other soils in which a considerable 

 amount of salt prevents the growth of plants, just as 

 we ]iut salt on grass to kill it. \'or are all regions in 

 which there is a heavy rainfall regions of luxuriant vege- 

 tation. In many cases the rainfall runs off so ra])idly, 

 or sinks so deeply into the soil, or evaporates so quickly 

 that plants can get little of it. In still other regions the 

 moisture may fall during the colder part of the year 

 instead of in the growing season. iMom these considera- 

 tions we perceive that a desert is not ])rimarily a region 

 of little water, but a region in which little water is avail- 

 able to plants. There are clearly two kinds of dry soils: 

 the physically dry soil that is actually dry and the physio- 

 logically dry soil that may have plenty of water and still 

 be dry to plants. In either case the conditions give rise 

 to desert vegetation. 



It is usually the physically dry soil that one thinks 

 of when the desert is mentioned. There are many such 

 in the warmer ]>arts of the earth. They are commonly 

 located behind mountain ranges that cut oft the moisture- 

 hearing winds, though sometimes that is due to the fact 

 that the prevailiiig winds blow from regions where there 

 is little moisture to begin with. The extreme type of 

 such an area in the cactus desert in which only the most 

 resistant i)lants like the cacti, yuccas, and agaves can 

 exist. The soil here is never covered with vegetation, 

 partly because of the difficulty new plants find in getting 

 started and jiartly because of the unfavorable conditions 

 for life afterwards. The soil itself is often quite fertile 

 and yields abundant crops when su]iplied with water 

 as we see in the irrigated regions of our Southwest. 

 Left to itself, however, vegetation assumes very curious 

 and interesting forms. The stems are either condensed 

 as in the various sjjecies of cactus, or underground as 

 in the yuccas. Leaves are usuallv small or absent and 

 the work of food-making is carried on by the stems. 

 ^^'hen the leaves are present, they are likely to resemble 

 those of the yucca with a hard thick epidermis that ef- 

 fectually retards evaporation. .\ few species produce 

 thinner leaves during the rainy season and droji them 

 Ijromptlv at the first signs of dessication. ".Switch- 

 plants" with slender leafless stems naturally abound. 



The roots of desert plants spread widely through the 

 soil ready to absorb any moisture that falls and not fre- 

 quently acting as storage organs for it. In a majoritv of 

 the plants, however, the storage organs are above .ground 

 in stem or leaves which are often greatly thickened for 

 the purpose with special cells for holding the water and a 

 mucilaginous juice to aid in preventing evaporation. 

 Nearly all desert plants have a grayish appearance which 

 may he due to a ])rotecting coating of hairs, scales or 

 waxy matter. The epidermis also may be thickened to 

 form a layer of cuticle and the breathing pores, or 

 stomata, are small and commonly sunk in the tissues 

 of the leaf. 



When the desert has a distinct rainy season, large 



numbers of short-lived annuals mav appear. These 

 spring up. ripen their seeds and comj)lete their life cycle 

 before dryness overtakes them. .Such species are usually 

 "rosette-plants" with leaves radiating in all directions 

 from the top of the stem which does not rise above the 

 soil. The dandelion is a good illustration of a rosette- 

 plant in more hospitable regions. Other species with a 

 longer term of life avoid the extreme heat and drouth 

 by casting off their aerial parts during the dry season 

 are retreating under ground, as it were. These are 

 known as "geophilous ]jlants." A few mosses, lichens 

 and fernworts simply dry up until a moi.st season returns 

 when they resume growth again. Some of these are 

 sold as curios under the name of "resurrection plants." 



Extreme dryness, is not the only noticeable character- 

 istic of the desert. ( hving to the lack of moisture, 

 clouds are few and the insolation great but the heat is 

 not oppressive because of the general dryness. Because 

 of the absence of moisture, also, the air and soil cool 

 very rapidly after sunset and may become decidedly 

 chilly before morning, even in the height of Smnmer. 

 The great differences in temperature that thus develop 

 cause high winds which blow the sand into hillocks and 

 dunes leaving large areas absolutely bare of vegetation. 

 .Sand-storms, that oblige one to suspend all travel until 

 thev pass, frequently occur. The water-courses may be 

 numerous, but thev seldom contain water except im- 

 mediately after a rain. Here and there in the desert, the 

 water comes to the .surface in springs or seeps and the 

 adjacent area takes on some of the aspects of more 

 fertile regions. Elsewhere the plants form low grounded 

 clumps and give the whole landscape a tufted api^ear- 

 ance. 



In desert regions with a distinct r.ainy period, the cac- 

 tus desert gives place to verv ditTerent forms of plants 

 known as sclcroplivlls. with small hard leaves which may 

 be retained throughout the year. The well-known grease- 

 wood, rabbit-lirusb ;'nd sage-brush, and various species 

 of Atriplcx. form the major part of such vegetation 

 forms though there are survivals from the cactus desert 

 such as the pricklv pear, numerous yuccas, and the ever- 

 present ]\Tormon-tea or joint-fir. In the direction of 

 heavier rainfall this sort of desert merges into the j'jlains 

 and often contains a number of hardier species of other 

 regions such as the thistle, ttniible-weed. evening jirim- 

 rose, milkweed, sunflowers and other composites. The 

 one-seeded juniper and the piiion pine may be foimd 

 in scattered clumps but everywhere the woody vegeta- 

 tion has a stmited and starved appearaijce with an 

 abundance of thorns and prickles. 



Coming to the deserts of moist regions we find excellent 

 examples in the dunes which the wind often piles uji in 

 sandv places. These are deserts due in part to the 

 sterility of the soil and in part to the fact that the rain- 

 fall soaks into the soil so quickly. The interior of the 

 dune is often, perhaps always, moist and dune plants 

 have long roots adapted to securing the moisture from 

 an extensive area. A pectdiar flora characterizes the 

 dunes. Among tvpical plants are the sand violet, the 

 hoarv pea. the huckleberi-y, wintergreen. sand cherry, 

 bavberry. sweet fern, bear berry, bnlterfly-weed, beach 

 grass and such trees as cottonwood, jack pine and black 

 oak. Rosette plants and switch plants are common and 

 mingled with the other vegetation the prickly pear per- 

 sists. 



The cliff is another kind of ;\ dc'^ert due to its general 



