Julius von Sachs (1832–1897) was a German botanist who developed a lot of thinking around plant physiology. This blog summarises (in my words) his writings on mistletoes from lectures written in 1887:
Aerial roots
Can be regarded as ordinary roots that are prevented from normal development by accidental circumstances. They are capable, when presented with soil, of growing into normal long, branched roots. These roots often remain short and simple when growing without soil.
Haustoria
Parasites have degraded root systems that "lose their ordinary typical structure, and at last only amorphous masses of tissues or even isolated cells remain". Haustoria only have in common with normal roots the fact that they penetrate substratum to absorb food.
Mistletoe (referring to Viscum species)
Mistletoes produce organic materials independently of the host tree with no lack of chlorophyll as even the fleshy roots within the host tree are green in colour. Parasitism by mistletoes is only partial and therefore so is the degradation for the host tree.
Aerial roots
Can be regarded as ordinary roots that are prevented from normal development by accidental circumstances. They are capable, when presented with soil, of growing into normal long, branched roots. These roots often remain short and simple when growing without soil.
Haustoria
Parasites have degraded root systems that "lose their ordinary typical structure, and at last only amorphous masses of tissues or even isolated cells remain". Haustoria only have in common with normal roots the fact that they penetrate substratum to absorb food.
Mistletoe (referring to Viscum species)
Mistletoes produce organic materials independently of the host tree with no lack of chlorophyll as even the fleshy roots within the host tree are green in colour. Parasitism by mistletoes is only partial and therefore so is the degradation for the host tree.
Lower portion of the stem (a) of the Mistletoe (Viscum album). h the wood of the shoot-axis; i primary root; ff roots growing in the cortex of the host-branch (c); g two buds arising from these; ee so-called Haustoria, roots which penetrate through the cambium into the young wood, and become surrounded by it later; bb wood of host-branch (half cut across at dd) showing the annual rings (nat. size). This is the original caption.
Germination and growth
When a viscid fruit sticks to a young tree branch, the large embryo, very rich in chlorophyll, germinates and its root tips turns away from the light, toward the host branch. The radicle bores through the cortex and cambium into the host wood. From these very active roots spring new shoots which break through the cortex of the host branch, come forth into the light and from these, new roots again run into the cortex. Occasionally, an entire tree, from crown to root, is infested with the mistletoe.
When a viscid fruit sticks to a young tree branch, the large embryo, very rich in chlorophyll, germinates and its root tips turns away from the light, toward the host branch. The radicle bores through the cortex and cambium into the host wood. From these very active roots spring new shoots which break through the cortex of the host branch, come forth into the light and from these, new roots again run into the cortex. Occasionally, an entire tree, from crown to root, is infested with the mistletoe.
Julius von Sachs. Source: Wikipedia.
So there you go, the study of these interesting plants has been happening for a long time!
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