FORM-AI.TERATIONS AND GROWTH OF CACTI. 29 



an addition was made to the laboratory; a stone wall and cement gutter 

 were built over the roots of the plant, and one large root was cut off. For 

 a time it was very much shriveled up, but it soon put out new roots and 

 recovered; yet the following year it grew but 5.5 cm. , while in the next two 

 years it gained 22 cm., or an average of 11 cm. each year. So far as this 

 evidence is of value it indicates that growth in height may be checked by 

 a lack of water. The enormous development of the storage system of 

 the sahuaro might, however, render the plant in good measure indepen- 

 dent of ordinary seasonal vicissitudes in the matter of growth. 



Growth in Circtjmpesence. 



From simple inspection of young and old sahuaros it is evident that the 

 trunk grows in thickness for a period of years, after which it maintains a 

 very nearly exact columnar form. The base of a sahuaro trunk, for a foot 

 more or less from the ground, is usually distinctly smaller than any other 

 part, and its diameter may steadil> increase from below upward (plate 5) 

 until the plant attains a height of 6 feet or more. This is because growth 

 in this species is apical, and the increments of each succeeding year in- 

 crease in diameter until the trunk attains its full thickness, after which it 

 maintains through its life a substantially uniform size. 



At the base of all but the very young plants there is always a brown 

 portion of the trunk which extends upward as the age of the plant increases. 

 Here the ribs and furrows are sometimes hardly discernible, and the sur- 

 face is rough and suberized. The tissues, too, from circumference to cen- 

 ter-are thoroughly hardened. Along the irregular boundary between this 

 and the green part of the trunk longitudinal cracks often appear in the 

 green shell of epidermis and mechanical tissue; the edges of the crack 

 draw apart for some time, perceptibly widening the furrow in which they 

 occur, and finally the whole is overlaid by suberized tissue. The repeti- 

 tion of this process has its share in causing the brown appearance of the 

 base of the trunk. This phenomenon of cracking and the fact that in the 

 older plants the furrows for the first few feet are often almost flattened out 

 are indications of a slight growth in circumference at the base of the trunk. 



If the green trunk of the sahuaro increases in circumference after it attains 

 its columnar form it should be evident by an increase in the size of the 

 intervals from year to year; but of 84 intervals measured for four successive 

 years, from 1903 to 1907, only 10 attained a greater width in 1907 than at 

 any other time; 8 of these 10 were on a small cactus, measuring a little 

 more than 3 feet high; 4 were so close to the top that the furrows in which 

 they were located would expand with the elongation of the trunk; 2 were 

 close to the base, and but 1 in the middle of the trunk. It is plain that if 

 growth occurs in the diameter of the main body of the sahuaro it is exceed- 

 ingly slow, especially after the plant has attained 4 or 5 feet in height. 



