SCIENCE-GOSSIP. 



331 



Pui-thei', it would be a decided advantage if the 

 plant was able to store vip water in its tissues for 

 its use in times of excessive drought. These two 

 facts furnish the clue to the whole phenomenon ; 

 all the variations in the vegetative structure are 

 means towards this end. For the prevention of 

 •excessive transpiration in these plants, we almost 

 iiniversally find a very thick cuticle and stomata 

 sunk below the surface. A coating of wax which 

 is frequently present on stems and leaves^ is 

 another means towards this end. The water- 

 storage is provided for by the thick, fleshy leaves 

 and stems, the central portions of which consist 

 of coloiu'less 

 cells containing 

 aqueous fluid. 



These points 

 may be illus- 

 trated more 

 clearly if I refer 

 to some ex- 

 amples which 

 show how svich 

 organs as leaves 

 whose functions 

 are essentially 

 transpiratory 

 and assimila- 

 tory, are var- 

 iously modifled, 

 and even super- 

 seded, appear 

 ing only as 

 spines (Opur- 

 tia), bristles 

 (Epiphyllum), 

 orminute scales 

 (Asparagus). 

 Correlatedwith 

 the reduction 

 of foliage, we 

 find an exten- 

 sion of stem- 

 structure, be- 



coming wing- 

 ed, flattened. 



J. Gregory 



View in Succulent 



ledonous trees — trees, that is, which could not 

 exist unless the rainfall was fairly liberal. Thus 

 we see that these plants are structurally correlated 

 with their more genial environment. 



Next we will consider the typical desert forms, 

 and in these we shall quickly observe highly 

 specialised modifications. In Opuntia the stems 

 are .flattened and articulated ; inlMamillaria, thick 

 and cylindrical, covered with conical protu- 

 berances (mamillae), terminated with a cluster of 

 spines. In all the other genera, they are more or 

 less triangular or columnar. All of them are 

 variously ridged or grooved, and vary from an 



inch (Cereus 

 rostratus), to 

 two feet or 

 more in dia- 

 meter, as in O. 

 (liganteus. The 

 bulk of the 

 tissue serves as 

 a reservoir for 

 water, assimi- 

 lation being- 

 relegated to the 

 peripheral lay- 

 ers of cells. 

 These stems are 

 armed with 

 spines of vary- 

 ing size, giving 

 them a very 

 formidable ap- 

 pearance. In 

 Echinocactus 

 ivislizeni they 

 are nearly four 

 inches long, 

 bent round at 

 their apical 

 portion in the 

 shape of a fish- 

 hook, hence the 

 name " fish- 

 hook cactus." 

 The spines not 



Photo. Croydon. 



columnar^ candelabra, and some other forms. 



In the Cactaceae, beginning with the genus 

 PeresMa, we] find a normal, woody stem clothed 

 with venomous spines, and producing fairly normal 

 leaves. In Epiphyllum, the stem is fiattened and 

 articulated, the leaves being represented by a few 

 bristles. In Phyllocactus, the stems are fiattened, 

 and free from the slightest vestige of a leaf. In 

 Bhipsalis, some species have fiattened, others 

 cylindrical stems. In all the above genera, the 

 stems are only slightly succulent as compared 

 with their desert allies, for the simple reason that, 

 with 'the exception of Pereslcia, they are all epi- 

 phytes, growing on ordinary, broad-leaved, dicoty- 



HOUSE AT KeW. 



only protect them from herbivorous animals, but 

 aid in breaking the incidence of the sun's rays. 



In the old-man cactus (Cereus senilis), and the 

 grizzly-bear opuntia (Opuntia senilis) , the whole 

 stem, and particularly the apical portion, is 

 thickly covered with long, silvery-grey hair. The 

 Turk's-cap cactus (Melocactus communis), is very 

 interesting, the top of the stems being crowned 

 by a big, shaggy cap. In large specimens this 

 may attain a size of two feet in circumference. It 

 is formed by the aggregation and persistence of 

 tufts of wooUy hair which are developed around 

 the ovary of each flower. 



(To he continued.) 



