90 THE AMERICAN BOTANIST. 



mal that the cactus cannot repel with spines. \\'hen thirsty 

 he is not averse to slicing off the top of the bisnaga (Echino- 

 cacfiis Enioryi) and obtaining a good drink of not unpalata- 

 ble water. There are other cacti, however, with spines small 

 or absent entirely and at first glance one is inclined to wonder 

 why these have not long ago been devoured. Investigation 

 shows that their immunity is due to the possession of bitter 

 or poisonous juices. These latter species, then are even more 

 successful than the spiny forms for they protect their juices 

 from even the lord of creation himself. 



Drouth and Cold. — The physiological effects of drouth 

 and cold are ver}' similar. Drouth hastens the fall of the leaf 

 just as cold does. Plants, such as pines, which do not cast 

 off their leaves in winter, are often found both in cold and 

 warm climates and in both they have the same needle-like 

 leaves adapted to retard transpiration. A search for the cause 

 of such adaptations in different climates reveals the fact that 

 the southern pines dwell in soils that are physically dry, while 

 the northern pines grow in soils that for part of the year are 

 physiologically dry through cold. Plants are as likely to 

 cast their leaves in a dry season in the tropics as they are at 

 the approach of winter in the temperate zones. 



Origin of Floral Numbers. — Take a straight vigorous 

 twig of any alternate leaved plant and beginning at one of the 

 lower nodes, pass a piece of twine from leaf to leaf up the 

 stem. In all normal specimens it will be found that the 

 leaves have a very definite position on the stem. The twine 

 may pass one or more times around the stem before coming 

 to a second leaf that is exactly over the one selected at the 

 beginning, but in all cases, the leaves are arranged alike in the 

 same species. In a large number of cases the twine goes round 

 the stem twice and passes five leaves or buds before coming to 



