164 



PLANT PHYSIOLOGY AND ECOLOGY 



6. The thorn form. This is typical of many spiny desert 

 shrubs, in which the leaves are lost very early, or are reduced 

 to mere functionless scales. The stems have an extremely thick 

 cuticle, and as a rule the stomata are deeply sunken and protected 

 by valves. Colletia and Holacantha are good examples of this 

 type. 



7. The succulent form. Plants with succulent stems such as 

 Euphorbia, Stapelia, and the Cactacece have decreased water 

 loss both by the extreme reduction or loss of leaves, and by reduc- 

 tion of the stem surface. In addition they guard against exces- 

 sive transpiration by means of water-storage tissues containing a 

 mucilaginous sap. The cuticle is usually highly developed and 

 the stomata sunken. Thorns and spines are also more or less 

 characteristic, though they serve only slightly and incidentally 

 against water loss. 



Experiment 50. Form and structure of stem xerophytes. Draw in 

 careful outline and to scale the stem or shoot of a representative plant 

 of each type. Make a drawing of a cross-section of the stem of the 

 virgate, or the rush form, and compare with the needle leaf. 



183. Mesophytes. j\Iesophytic species grow in habitats that 

 are neither extremely dry nor wet, and consequently they show 

 no striking response to water supply or loss. They possess a 

 form or structure that is more or less characteristic by reason of 



Fig. 54. Cross-section of the leaf of a mesophyte, Pedicularis procera. 



the absence of distinct modifications. As their name indicates, 

 mesophytes are middle plants, i.e., they stand midway between 

 xerophytes and hydrophytes. For this reason, they pass on the 

 one hand into dry land plants, and on the other into water plants. 

 More than this, the less intense xerophytes and hydrophytes have 



