A Tropical Spring 



by William C. Burger, 



Assistant Curator, Vascular Plants 



The skies have been clear for almost five months, with only 

 an occasional cloud formation and a rare shower or two. 

 The earth is parched; almost all the plants are leafless. 

 The wind, dry and dust-laden, has blown steadily from 

 the north and east. But now there is a change : the winds 

 are shifting, and soon they will sweep in from the south. 

 Fluffy clouds begin to form and then develop into thunder- 

 heads. The air is becoming sweet with moisture. The 

 monsoon has begun, and with the advent of this rainy 

 season there comes a tropical spring. 



Many people think of the tropics as a steaming jungle 

 where luxuriant vegetation prospers throughout the year; 

 however, these areas are in the minority. Most tropical 

 regions experience a dry season for at least part of the year. 

 The duration of the dry period determines whether an 

 area has a tall evergreen forest, a deciduous forest, or a 

 sub-desert thorn-scrub. The area in eastern Ethiopia that 

 I am familiar with has a dry period of nearly six months, 

 and here the rains support a vegetation of deciduous trees. 

 These are trees like our own that lose their leaves at the end 

 of the growing season. 



In a way, the monsoon, seasonally wet and dry or 

 tropical climate, is similar to our own in the temperate zone. 

 The dry season is comparable to our winter, and our sum- 

 mer is similar to the growing or wet period of the seasonally 

 dry tropics. In these areas even the size of trees and density 

 of vegetation may look similar to ours. For plants, winter 

 and the dry season are very much alike : periods when physi- 

 ological processes stop or are severely limited . Cold weather 

 prevents the plant from moving water rapidly through its 

 tissues, producing a physiological drought not unlike a cli- 

 matic drought. For animals, too, winter and the dry season 

 are the times of food scarcity. 



The end of the dry period and the coming of the rains 

 usher in a new cycle of growth and activity, it is a tropical 

 springtime. Like our spring it heralds the blooming of 

 flowers and leafing of trees, the nesting of birds and the 

 emergence of insects. The comparison of temperate and 

 tropical "spring" is not an unreasonable consideration. 

 Plants and animals have adjusted in much the same fashion 

 to the exigencies of winter and of drought. When the cold 

 period or the dry period ends, the responses of living things 

 are also similar. For plants, as for animals, survival through 

 a long cold or dry period requires the storage of food. 

 Whether in seeds, underground bulbs or roots, the plant 

 must have food to carry it through the dry period and per- 

 mit the resumption of active growth in springtime. Perhaps 

 the most spectacular plants of springtime are those that ex- 

 hibit this point best: plants such as the tulip, hyacinth and 

 iris. These have an abundant undergroimd store of food 

 which permits them to produce a large and brilliant floral 



Page 10 APRIL 



Crinum in flower. Photos taken in eastern Ethiopia 

 by the author. 



display at the beginning of the growing season. Energy is re- 

 quired to produce a large cluster of flowers, and this energy 

 comes from the food produced during the previous growing 

 season and stored over the winter. Similarly, some of the 

 most spectacular flowers of the seasonally dry African trop- 

 ics are those related to tulips and irises. These, too, have 

 underground storage organs, and they also have the ability 

 to produce an extraordinary floral display in a short period 

 of time. In only two weeks Haemanthus, Crinum, and many 

 lily-like plants can produce a cluster of flowers remarkable 

 in size and number. Likewise, many trees of the tropics will 

 come into flower at the beginning of the rains, not unlike 

 our redbuds, dogwoods, and fruit trees. These, too, have 

 food stored in their roots from the previous growing season. 



Haemanthus multiflorus, a blood lily, in full flower while 

 ntker plants around it have only begun to sprout. 



