AN AVOCADO MONSTROSITY 



Robert W. Hodgson 

 University of Calif ornia,- Berkeley, Cal. 



WITH the rapid extension of 

 avocado planting in California 

 and the consequent demand for 

 trees, nurser^nnen have had oc- 

 casion to raise many thousands of 

 seedlings during the last four or five 

 years. Among such a large number 

 naturally some abnormal forms have 

 appeared such as albinos and dwarfs, 

 and among others a few teratological 

 monstrosities have been reported. The 

 latter usually take the form of fasciation 

 in which terminal nodes have become 

 expanded, presenting a ridged, flat stem 

 of greatly increased width upon which 

 normal buds are borne. Among such 

 peculiar forms none more striking has 

 been reported than that pictured in 

 Fig. 7 which was found in a lot of 

 Grande seedlings in the University 

 greenhouse. This particular case does 

 not exhibit the usual characteristics 

 of fasciation and seems worthy of being 

 put on record. 



The peculiar growth was first noticed 

 when the seedling was about seven 

 months old and is still present. Al- 

 though the first leaves were shed some 

 time ago, the new growth exhibits the 

 same peculiarity. The malformation 

 consists of the hypertrophy of the termi- 

 nal cluster of leaves together with several 

 adjacent nodes. The leaves are mark- 

 edly modified both in form, size, and 

 consistency. The size is greatly re- 

 duced, and the terminal node is so 

 shortened that the leaves are crowded 

 together much as in the case of a brussels 

 sprout. The consistency and form of 

 the leaves are so modified that the 

 general appearance is that of a much 

 corrugated, imbricated mass of white 

 fleshy tissue possessing a shiny surface. 

 Very careful examination revealed no 

 traces of the presence of insects or of 

 wounds caused by them. Microscopical 

 examination has failed to show any in- 

 dication of fungi. The general appear- 



ance of the modified leaves suggested 

 a possible connection between them and 

 the large fleshy cotyledons, but study 

 has failed to confirm this. The cotyle- 

 don tissue was found to contain very 

 large quantities of starch, some sugar, 

 and notable amounts of oil. The ab- 

 normal tissue contained little of these 

 but was fairly rich in protein. The cells 

 of the cotyledon tissue were found to 

 be heavy walled, larger, and in other 

 ways distinctly different from the spongy 

 pith-like, thin walled tissue of the modi- 

 fied leaves. Sections through such leaves 

 showed that they lacked the palisade 

 layer and spongy parenchyma entirely 

 and possessed but a rudimentary vascu- 

 lar system. A few cells containing 

 chloroplastids were found scattered 

 here and there. 



The effect of the abnormality on the 

 growth of the seedling has been such 

 that a shoot from a node further down 

 has developed and has taken the as- 

 cendency. This fact together with the 

 fact that all subsequent growth of the 

 affected nodes has been of the abnormal 

 type seems to indicate that the stimulus 

 or change resulting in the peculiar 

 growth has probably not been of a 

 momentary nature such as the sting 

 of an insect. To be sure, it may be 

 possible that the abnormal tissue in this 

 case is analagous to new foiTns which 

 have been produced by the injection of 

 certain chemicals into the ovaries of 

 plants as established by Lloyd and Mac- 

 Dougal, but in this case the change has 

 been purely somatic. 



Examination of the literature does 

 not discover much that is helpful in 

 classifying this peculiarity and indicates 

 that our present knowledge as to the 

 cause of teratological forms is quite 

 unsatisfactory. Thorough study of such 

 forms might help unravel the mystery 

 concerning the causes leading up to 

 bud-mutations. 



557 



