298 DISEASES OF PLANTS. 



with this metamorphosis ; as they appear to be more intimate- 

 ly connected with the petals than the pistils. 



5th. The petioles or foot stalks often change to leaves. 

 This may be seen in an Arabian plant ACACIA zilotica, which 

 furnishes the gum arabic. This tree at first exhibits upon one 

 petiole six or eight pair of leaves ; this number every year 

 becomes less, until all the leaves disappear ; the petiole then 

 retaining all the nourishment which before was distributed to 

 the leaves, flattens and expands, and appears in the form of a 

 thick leaf. The trees which we call Acacia are not of this 

 genus, but of the genus Robinia. 



6th. The peduncles and petioles, sometimes change into 

 tendrils, as in the vine ; this plant at first throws out many 

 large leaves and clusters of flowers ; but the food not being 

 sufficient to support such a profuse vegetation, the new leaves 

 and clusters of flowers appear smaller ; the nourishment be- 

 coming still more scanty, at length neither flower or leaf is 

 developed, and the peduncle and petiole become tendrils, which 

 by attaching themselves to some firm bodies, serve to sustain 

 the rich fruit which is perfected on the flower parts of the 

 branch, 



7th. The last change we shall notice is the transformation 

 of buds into thorns. When a plant forms more buds than it 

 can nourish, some of them do not develope branches and leaves, 

 but becoming hardened by the accumulation of sap, which is 

 insufficient for their full perfection, they exhibit the short 

 indurated process called a thorn. It is said that wild plants by 

 rich cultivation, do in time become divested of their thorns, 

 which change into what they seemed originally destined for, 

 viz. leaves and branches. 



Prickles, such as may be seen upon the rose, gooseberry 

 and other plants, do not change by cultivation, for these are a 

 natural appendage, originating from the bark ; while the thorn 

 may be found connected with the wood, of which it seems to 

 make a part. 



Diseases of Plants.* 



The diseases of plants, for these organized beings are, like 

 animals, subject to disease and death, may in many cases arise 

 from causes within the knowledge of the attentive naturalist. 



1st. We notice constitutional diseases. Of this class are 



* This constitutes a department of Botany called pathology ; a term derived 

 from two Greek words, pathos, disease, and logos, account. 



5th. Changes in petioles 6th. Peduncles and petioles become vines 7th. 

 Buds, how transformed Prickles Diseases of plants 1st. Constitutional. 



